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))
Follow-up to https://github.com/element-hq/synapse/pull/18604
Previously, our cache metrics did include the `server_name` label as
expected but we were only seeing the last server being reported. This
was caused because we would
`CACHE_METRIC_REGISTRY.register_hook(metric_name, metric.collect)` where
the `metric_name` only took into account the cache name so it would be
overwritten every time we spawn a new server.
This PR updates the register logic to include the `server_name` so we
have a hook for every cache on every server as expected.
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.
Part of https://github.com/element-hq/synapse/issues/18592
Separated out of https://github.com/element-hq/synapse/pull/18656
because it's a bigger, unique piece of the refactor
### Testing strategy
1. Add the `metrics` listener in your `homeserver.yaml`
```yaml
listeners:
# This is just showing how to configure metrics either way
#
# `http` `metrics` resource
- port: 9322
type: http
bind_addresses: ['127.0.0.1']
resources:
- names: [metrics]
compress: false
# `metrics` listener
- port: 9323
type: metrics
bind_addresses: ['127.0.0.1']
```
1. Start the homeserver: `poetry run synapse_homeserver --config-path
homeserver.yaml`
1. Fetch `http://localhost:9322/_synapse/metrics` and/or
`http://localhost:9323/metrics`
1. Observe response includes the background processs metrics
(`synapse_background_process_start_count`,
`synapse_background_process_db_txn_count_total`, etc) with the
`server_name` label
Clean up `MetricsResource`, Prometheus hacks
(`_set_prometheus_client_use_created_metrics`), and better document why
we care about having a separate `metrics` listener type.
These clean-up changes have been split out from
https://github.com/element-hq/synapse/pull/18584 since that PR was
closed.
Reverts element-hq/synapse#18260
It is causing a failure when building release debs for `debian:bullseye`
with the following error:
```
sqlite3.OperationalError: near "RETURNING": syntax error
```
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
We change it so that each cache has an individual CacheMetric, instead
of having one global CacheMetric. This means that when a cache tries to
increment a counter it does not need to go through so many indirections.
