Fix LaterGauge metrics to collect from all servers (#18751)

Fix `LaterGauge` metrics to collect from all servers

Follow-up to https://github.com/element-hq/synapse/pull/18714

Previously, our `LaterGauge` metrics did include the `server_name` label
as expected but we were only seeing the last server being reported in
some cases. Any `LaterGauge` that we were creating multiple times was
only reporting the last instance.

This PR updates all `LaterGauge` to be created once and then we use
`LaterGauge.register_hook(...)` to add in the metric callback as before.
This works now because we store a list of callbacks instead of just one.

I noticed this problem thanks to some [tests in the Synapse Pro for
Small Hosts](https://github.com/element-hq/synapse-small-hosts/pull/173)
repo that sanity check all metrics to ensure that we can see each metric
includes data from multiple servers.


### Testing strategy

1. This is only noticeable when you run multiple Synapse instances in
the same process.
 1. TODO

(see test that was added)

### Dev notes

Previous non-global `LaterGauge`:

```
synapse_federation_send_queue_xxx
synapse_federation_transaction_queue_pending_destinations
synapse_federation_transaction_queue_pending_pdus
synapse_federation_transaction_queue_pending_edus
synapse_handlers_presence_user_to_current_state_size
synapse_handlers_presence_wheel_timer_size
synapse_notifier_listeners
synapse_notifier_rooms
synapse_notifier_users
synapse_replication_tcp_resource_total_connections
synapse_replication_tcp_command_queue
synapse_background_update_status
synapse_federation_known_servers
synapse_scheduler_running_tasks
```



### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/18751.misc

@@ -0,0 +1 @@
+Fix `LaterGauge` metrics to collect from all servers.

synapse/federation/send_queue.py

@@ -37,6 +37,7 @@ Events are replicated via a separate events stream.
 """
 
 import logging
+from enum import Enum
 from typing import (
     TYPE_CHECKING,
     Dict,
@@ -67,6 +68,25 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
+class QueueNames(str, Enum):
+    PRESENCE_MAP = "presence_map"
+    KEYED_EDU = "keyed_edu"
+    KEYED_EDU_CHANGED = "keyed_edu_changed"
+    EDUS = "edus"
+    POS_TIME = "pos_time"
+    PRESENCE_DESTINATIONS = "presence_destinations"
+
+
+queue_name_to_gauge_map: Dict[QueueNames, LaterGauge] = {}
+
+for queue_name in QueueNames:
+    queue_name_to_gauge_map[queue_name] = LaterGauge(
+        name=f"synapse_federation_send_queue_{queue_name.value}_size",
+        desc="",
+        labelnames=[SERVER_NAME_LABEL],
+    )
+
+
 class FederationRemoteSendQueue(AbstractFederationSender):
     """A drop in replacement for FederationSender"""
 
@@ -111,23 +131,15 @@ class FederationRemoteSendQueue(AbstractFederationSender):
         # we make a new function, so we need to make a new function so the inner
         # lambda binds to the queue rather than to the name of the queue which
         # changes. ARGH.
-        def register(name: str, queue: Sized) -> None:
+        def register(queue_name: QueueNames, queue: Sized) -> None:
-            LaterGauge(
+            queue_name_to_gauge_map[queue_name].register_hook(
-                name="synapse_federation_send_queue_%s_size" % (queue_name,),
+                lambda: {(self.server_name,): len(queue)}
-                desc="",
-                labelnames=[SERVER_NAME_LABEL],
-                caller=lambda: {(self.server_name,): len(queue)},
             )
 
-        for queue_name in [
+        for queue_name in QueueNames:
-            "presence_map",
+            queue = getattr(self, queue_name.value)
-            "keyed_edu",
+            assert isinstance(queue, Sized)
-            "keyed_edu_changed",
+            register(queue_name, queue=queue)
-            "edus",
-            "pos_time",
-            "presence_destinations",
-        ]:
-            register(queue_name, getattr(self, queue_name))
 
         self.clock.looping_call(self._clear_queue, 30 * 1000)
 

synapse/federation/sender/__init__.py

@@ -199,6 +199,24 @@ sent_pdus_destination_dist_total = Counter(
     labelnames=[SERVER_NAME_LABEL],
 )
 
+transaction_queue_pending_destinations_gauge = LaterGauge(
+    name="synapse_federation_transaction_queue_pending_destinations",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+transaction_queue_pending_pdus_gauge = LaterGauge(
+    name="synapse_federation_transaction_queue_pending_pdus",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+transaction_queue_pending_edus_gauge = LaterGauge(
+    name="synapse_federation_transaction_queue_pending_edus",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
 # Time (in s) to wait before trying to wake up destinations that have
 # catch-up outstanding.
 # Please note that rate limiting still applies, so while the loop is
@@ -398,38 +416,28 @@ class FederationSender(AbstractFederationSender):
         # map from destination to PerDestinationQueue
         self._per_destination_queues: Dict[str, PerDestinationQueue] = {}
 
-        LaterGauge(
+        transaction_queue_pending_destinations_gauge.register_hook(
-            name="synapse_federation_transaction_queue_pending_destinations",
+            lambda: {
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {
                 (self.server_name,): sum(
                     1
                     for d in self._per_destination_queues.values()
                     if d.transmission_loop_running
                 )
-            },
+            }
         )
-
+        transaction_queue_pending_pdus_gauge.register_hook(
-        LaterGauge(
+            lambda: {
-            name="synapse_federation_transaction_queue_pending_pdus",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {
                 (self.server_name,): sum(
                     d.pending_pdu_count() for d in self._per_destination_queues.values()
                 )
-            },
+            }
         )
-        LaterGauge(
+        transaction_queue_pending_edus_gauge.register_hook(
-            name="synapse_federation_transaction_queue_pending_edus",
+            lambda: {
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {
                 (self.server_name,): sum(
                     d.pending_edu_count() for d in self._per_destination_queues.values()
                 )
-            },
+            }
         )
 
         self._is_processing = False

synapse/handlers/presence.py

@@ -173,6 +173,18 @@ state_transition_counter = Counter(
     labelnames=["locality", "from", "to", SERVER_NAME_LABEL],
 )
 
+presence_user_to_current_state_size_gauge = LaterGauge(
+    name="synapse_handlers_presence_user_to_current_state_size",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+presence_wheel_timer_size_gauge = LaterGauge(
+    name="synapse_handlers_presence_wheel_timer_size",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
 # If a user was last active in the last LAST_ACTIVE_GRANULARITY, consider them
 # "currently_active"
 LAST_ACTIVE_GRANULARITY = 60 * 1000
@@ -779,11 +791,8 @@ class PresenceHandler(BasePresenceHandler):
             EduTypes.PRESENCE, self.incoming_presence
         )
 
-        LaterGauge(
+        presence_user_to_current_state_size_gauge.register_hook(
-            name="synapse_handlers_presence_user_to_current_state_size",
+            lambda: {(self.server_name,): len(self.user_to_current_state)}
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): len(self.user_to_current_state)},
         )
 
         # The per-device presence state, maps user to devices to per-device presence state.
@@ -882,11 +891,8 @@ class PresenceHandler(BasePresenceHandler):
                 60 * 1000,
             )
 
-        LaterGauge(
+        presence_wheel_timer_size_gauge.register_hook(
-            name="synapse_handlers_presence_wheel_timer_size",
+            lambda: {(self.server_name,): len(self.wheel_timer)}
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): len(self.wheel_timer)},
         )
 
         # Used to handle sending of presence to newly joined users/servers

synapse/http/request_metrics.py

@@ -164,12 +164,12 @@ def _get_in_flight_counts() -> Mapping[Tuple[str, ...], int]:
     return counts
 
 
-LaterGauge(
+in_flight_requests = LaterGauge(
     name="synapse_http_server_in_flight_requests_count",
     desc="",
     labelnames=["method", "servlet", SERVER_NAME_LABEL],
-    caller=_get_in_flight_counts,
 )
+in_flight_requests.register_hook(_get_in_flight_counts)
 
 
 class RequestMetrics:

synapse/metrics/__init__.py

@@ -31,6 +31,7 @@ from typing import (
     Dict,
     Generic,
     Iterable,
+    List,
     Mapping,
     Optional,
     Sequence,
@@ -73,8 +74,6 @@ logger = logging.getLogger(__name__)
 
 METRICS_PREFIX = "/_synapse/metrics"
 
-all_gauges: Dict[str, Collector] = {}
-
 HAVE_PROC_SELF_STAT = os.path.exists("/proc/self/stat")
 
 SERVER_NAME_LABEL = "server_name"
@@ -163,42 +162,47 @@ class LaterGauge(Collector):
     name: str
     desc: str
     labelnames: Optional[StrSequence] = attr.ib(hash=False)
-    # callback: should either return a value (if there are no labels for this metric),
+    # List of callbacks: each callback should either return a value (if there are no
-    # or dict mapping from a label tuple to a value
+    # labels for this metric), or dict mapping from a label tuple to a value
-    caller: Callable[
+    _hooks: List[
-        [], Union[Mapping[Tuple[str, ...], Union[int, float]], Union[int, float]]
+        Callable[
-    ]
+            [], Union[Mapping[Tuple[str, ...], Union[int, float]], Union[int, float]]
+        ]
+    ] = attr.ib(factory=list, hash=False)
 
     def collect(self) -> Iterable[Metric]:
         # The decision to add `SERVER_NAME_LABEL` is from the `LaterGauge` usage itself
         # (we don't enforce it here, one level up).
         g = GaugeMetricFamily(self.name, self.desc, labels=self.labelnames)  # type: ignore[missing-server-name-label]
 
-        try:
+        for hook in self._hooks:
-            calls = self.caller()
+            try:
-        except Exception:
+                hook_result = hook()
-            logger.exception("Exception running callback for LaterGauge(%s)", self.name)
+            except Exception:
+                logger.exception(
+                    "Exception running callback for LaterGauge(%s)", self.name
+                )
+                yield g
+                return
+
+            if isinstance(hook_result, (int, float)):
+                g.add_metric([], hook_result)
+            else:
+                for k, v in hook_result.items():
+                    g.add_metric(k, v)
+
             yield g
-            return
 
-        if isinstance(calls, (int, float)):
+    def register_hook(
-            g.add_metric([], calls)
+        self,
-        else:
+        hook: Callable[
-            for k, v in calls.items():
+            [], Union[Mapping[Tuple[str, ...], Union[int, float]], Union[int, float]]
-                g.add_metric(k, v)
+        ],
-
+    ) -> None:
-        yield g
+        self._hooks.append(hook)
 
     def __attrs_post_init__(self) -> None:
-        self._register()
-
-    def _register(self) -> None:
-        if self.name in all_gauges.keys():
-            logger.warning("%s already registered, reregistering", self.name)
-            REGISTRY.unregister(all_gauges.pop(self.name))
-
         REGISTRY.register(self)
-        all_gauges[self.name] = self
 
 
 # `MetricsEntry` only makes sense when it is a `Protocol`,
@@ -250,7 +254,7 @@ class InFlightGauge(Generic[MetricsEntry], Collector):
         # Protects access to _registrations
         self._lock = threading.Lock()
 
-        self._register_with_collector()
+        REGISTRY.register(self)
 
     def register(
         self,
@@ -341,14 +345,6 @@ class InFlightGauge(Generic[MetricsEntry], Collector):
                 gauge.add_metric(labels=key, value=getattr(metrics, name))
             yield gauge
 
-    def _register_with_collector(self) -> None:
-        if self.name in all_gauges.keys():
-            logger.warning("%s already registered, reregistering", self.name)
-            REGISTRY.unregister(all_gauges.pop(self.name))
-
-        REGISTRY.register(self)
-        all_gauges[self.name] = self
-
 
 class GaugeHistogramMetricFamilyWithLabels(GaugeHistogramMetricFamily):
     """

synapse/notifier.py

@@ -86,6 +86,24 @@ users_woken_by_stream_counter = Counter(
     labelnames=["stream", SERVER_NAME_LABEL],
 )
 
+
+notifier_listeners_gauge = LaterGauge(
+    name="synapse_notifier_listeners",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+notifier_rooms_gauge = LaterGauge(
+    name="synapse_notifier_rooms",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+notifier_users_gauge = LaterGauge(
+    name="synapse_notifier_users",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
 T = TypeVar("T")
 
 
@@ -281,28 +299,16 @@ class Notifier:
                 )
             }
 
-        LaterGauge(
+        notifier_listeners_gauge.register_hook(count_listeners)
-            name="synapse_notifier_listeners",
+        notifier_rooms_gauge.register_hook(
-            desc="",
+            lambda: {
-            labelnames=[SERVER_NAME_LABEL],
-            caller=count_listeners,
-        )
-
-        LaterGauge(
-            name="synapse_notifier_rooms",
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {
                 (self.server_name,): count(
                     bool, list(self.room_to_user_streams.values())
                 )
-            },
+            }
         )
-        LaterGauge(
+        notifier_users_gauge.register_hook(
-            name="synapse_notifier_users",
+            lambda: {(self.server_name,): len(self.user_to_user_stream)}
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): len(self.user_to_user_stream)},
         )
 
     def add_replication_callback(self, cb: Callable[[], None]) -> None:

synapse/replication/tcp/handler.py

@@ -106,6 +106,18 @@ user_ip_cache_counter = Counter(
     "synapse_replication_tcp_resource_user_ip_cache", "", labelnames=[SERVER_NAME_LABEL]
 )
 
+tcp_resource_total_connections_gauge = LaterGauge(
+    name="synapse_replication_tcp_resource_total_connections",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+tcp_command_queue_gauge = LaterGauge(
+    name="synapse_replication_tcp_command_queue",
+    desc="Number of inbound RDATA/POSITION commands queued for processing",
+    labelnames=["stream_name", SERVER_NAME_LABEL],
+)
+
 
 # the type of the entries in _command_queues_by_stream
 _StreamCommandQueue = Deque[
@@ -243,11 +255,8 @@ class ReplicationCommandHandler:
         # outgoing replication commands to.)
         self._connections: List[IReplicationConnection] = []
 
-        LaterGauge(
+        tcp_resource_total_connections_gauge.register_hook(
-            name="synapse_replication_tcp_resource_total_connections",
+            lambda: {(self.server_name,): len(self._connections)}
-            desc="",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): len(self._connections)},
         )
 
         # When POSITION or RDATA commands arrive, we stick them in a queue and process
@@ -266,14 +275,11 @@ class ReplicationCommandHandler:
         # from that connection.
         self._streams_by_connection: Dict[IReplicationConnection, Set[str]] = {}
 
-        LaterGauge(
+        tcp_command_queue_gauge.register_hook(
-            name="synapse_replication_tcp_command_queue",
+            lambda: {
-            desc="Number of inbound RDATA/POSITION commands queued for processing",
-            labelnames=["stream_name", SERVER_NAME_LABEL],
-            caller=lambda: {
                 (stream_name, self.server_name): len(queue)
                 for stream_name, queue in self._command_queues_by_stream.items()
-            },
+            }
         )
 
         self._is_master = hs.config.worker.worker_app is None

synapse/replication/tcp/protocol.py

@@ -527,9 +527,11 @@ pending_commands = LaterGauge(
     name="synapse_replication_tcp_protocol_pending_commands",
     desc="",
     labelnames=["name", SERVER_NAME_LABEL],
-    caller=lambda: {
+)
+pending_commands.register_hook(
+    lambda: {
         (p.name, p.server_name): len(p.pending_commands) for p in connected_connections
-    },
+    }
 )
 
 
@@ -544,9 +546,11 @@ transport_send_buffer = LaterGauge(
     name="synapse_replication_tcp_protocol_transport_send_buffer",
     desc="",
     labelnames=["name", SERVER_NAME_LABEL],
-    caller=lambda: {
+)
+transport_send_buffer.register_hook(
+    lambda: {
         (p.name, p.server_name): transport_buffer_size(p) for p in connected_connections
-    },
+    }
 )
 
 
@@ -571,10 +575,12 @@ tcp_transport_kernel_send_buffer = LaterGauge(
     name="synapse_replication_tcp_protocol_transport_kernel_send_buffer",
     desc="",
     labelnames=["name", SERVER_NAME_LABEL],
-    caller=lambda: {
+)
+tcp_transport_kernel_send_buffer.register_hook(
+    lambda: {
         (p.name, p.server_name): transport_kernel_read_buffer_size(p, False)
         for p in connected_connections
-    },
+    }
 )
 
 
@@ -582,8 +588,10 @@ tcp_transport_kernel_read_buffer = LaterGauge(
     name="synapse_replication_tcp_protocol_transport_kernel_read_buffer",
     desc="",
     labelnames=["name", SERVER_NAME_LABEL],
-    caller=lambda: {
+)
+tcp_transport_kernel_read_buffer.register_hook(
+    lambda: {
         (p.name, p.server_name): transport_kernel_read_buffer_size(p, True)
         for p in connected_connections
-    },
+    }
 )

synapse/storage/database.py

@@ -100,6 +100,12 @@ sql_txn_duration = Counter(
     labelnames=["desc", SERVER_NAME_LABEL],
 )
 
+background_update_status = LaterGauge(
+    name="synapse_background_update_status",
+    desc="Background update status",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
 
 # Unique indexes which have been added in background updates. Maps from table name
 # to the name of the background update which added the unique index to that table.
@@ -611,11 +617,8 @@ class DatabasePool:
         )
 
         self.updates = BackgroundUpdater(hs, self)
-        LaterGauge(
+        background_update_status.register_hook(
-            name="synapse_background_update_status",
+            lambda: {(self.server_name,): self.updates.get_status()},
-            desc="Background update status",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): self.updates.get_status()},
         )
 
         self._previous_txn_total_time = 0.0

synapse/storage/databases/main/roommember.py

@@ -84,6 +84,13 @@ _CURRENT_STATE_MEMBERSHIP_UPDATE_NAME = "current_state_events_membership"
 _POPULATE_PARTICIPANT_BG_UPDATE_BATCH_SIZE = 1000
 
 
+federation_known_servers_gauge = LaterGauge(
+    name="synapse_federation_known_servers",
+    desc="",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+
 @attr.s(frozen=True, slots=True, auto_attribs=True)
 class EventIdMembership:
     """Returned by `get_membership_from_event_ids`"""
@@ -116,11 +123,8 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
                 1,
                 self._count_known_servers,
             )
-            LaterGauge(
+            federation_known_servers_gauge.register_hook(
-                name="synapse_federation_known_servers",
+                lambda: {(self.server_name,): self._known_servers_count}
-                desc="",
-                labelnames=[SERVER_NAME_LABEL],
-                caller=lambda: {(self.server_name,): self._known_servers_count},
             )
 
     @wrap_as_background_process("_count_known_servers")

synapse/util/ratelimitutils.py

@@ -131,27 +131,31 @@ def _get_counts_from_rate_limiter_instance(
 # We track the number of affected hosts per time-period so we can
 # differentiate one really noisy homeserver from a general
 # ratelimit tuning problem across the federation.
-LaterGauge(
+sleep_affected_hosts_gauge = LaterGauge(
     name="synapse_rate_limit_sleep_affected_hosts",
     desc="Number of hosts that had requests put to sleep",
     labelnames=["rate_limiter_name", SERVER_NAME_LABEL],
-    caller=lambda: _get_counts_from_rate_limiter_instance(
+)
+sleep_affected_hosts_gauge.register_hook(
+    lambda: _get_counts_from_rate_limiter_instance(
         lambda rate_limiter_instance: sum(
             ratelimiter.should_sleep()
             for ratelimiter in rate_limiter_instance.ratelimiters.values()
         )
-    ),
+    )
 )
-LaterGauge(
+reject_affected_hosts_gauge = LaterGauge(
     name="synapse_rate_limit_reject_affected_hosts",
     desc="Number of hosts that had requests rejected",
     labelnames=["rate_limiter_name", SERVER_NAME_LABEL],
-    caller=lambda: _get_counts_from_rate_limiter_instance(
+)
+reject_affected_hosts_gauge.register_hook(
+    lambda: _get_counts_from_rate_limiter_instance(
         lambda rate_limiter_instance: sum(
             ratelimiter.should_reject()
             for ratelimiter in rate_limiter_instance.ratelimiters.values()
         )
-    ),
+    )
 )
 
 

synapse/util/task_scheduler.py

@@ -44,6 +44,13 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
+running_tasks_gauge = LaterGauge(
+    name="synapse_scheduler_running_tasks",
+    desc="The number of concurrent running tasks handled by the TaskScheduler",
+    labelnames=[SERVER_NAME_LABEL],
+)
+
+
 class TaskScheduler:
     """
     This is a simple task scheduler designed for resumable tasks. Normally,
@@ -130,11 +137,8 @@ class TaskScheduler:
                 TaskScheduler.SCHEDULE_INTERVAL_MS,
             )
 
-        LaterGauge(
+        running_tasks_gauge.register_hook(
-            name="synapse_scheduler_running_tasks",
+            lambda: {(self.server_name,): len(self._running_tasks)}
-            desc="The number of concurrent running tasks handled by the TaskScheduler",
-            labelnames=[SERVER_NAME_LABEL],
-            caller=lambda: {(self.server_name,): len(self._running_tasks)},
         )
 
     def register_action(

tests/metrics/test_metrics.py

@@ -22,7 +22,13 @@ from typing import Dict, Protocol, Tuple
 
 from prometheus_client.core import Sample
 
-from synapse.metrics import REGISTRY, InFlightGauge, generate_latest
+from synapse.metrics import (
+    REGISTRY,
+    SERVER_NAME_LABEL,
+    InFlightGauge,
+    LaterGauge,
+    generate_latest,
+)
 from synapse.util.caches.deferred_cache import DeferredCache
 
 from tests import unittest
@@ -285,6 +291,42 @@ class CacheMetricsTests(unittest.HomeserverTestCase):
         self.assertEqual(hs2_cache_max_size_metric_value, "777.0")
 
 
+class LaterGaugeTests(unittest.HomeserverTestCase):
+    def test_later_gauge_multiple_servers(self) -> None:
+        """
+        Test that LaterGauge metrics are reported correctly across multiple servers. We
+        will have an metrics entry for each homeserver that is labeled with the
+        `server_name` label.
+        """
+        later_gauge = LaterGauge(
+            name="foo",
+            desc="",
+            labelnames=[SERVER_NAME_LABEL],
+        )
+        later_gauge.register_hook(lambda: {("hs1",): 1})
+        later_gauge.register_hook(lambda: {("hs2",): 2})
+
+        metrics_map = get_latest_metrics()
+
+        # Find the metrics for the caches from both homeservers
+        hs1_metric = 'foo{server_name="hs1"}'
+        hs1_metric_value = metrics_map.get(hs1_metric)
+        self.assertIsNotNone(
+            hs1_metric_value,
+            f"Missing metric {hs1_metric} in cache metrics {metrics_map}",
+        )
+        hs2_metric = 'foo{server_name="hs2"}'
+        hs2_metric_value = metrics_map.get(hs2_metric)
+        self.assertIsNotNone(
+            hs2_metric_value,
+            f"Missing metric {hs2_metric} in cache metrics {metrics_map}",
+        )
+
+        # Sanity check the metric values
+        self.assertEqual(hs1_metric_value, "1.0")
+        self.assertEqual(hs2_metric_value, "2.0")
+
+
 def get_latest_metrics() -> Dict[str, str]:
     """
     Collect the latest metrics from the registry and parse them into an easy to use map.