Fix InFlightGauge typing to allow upgrading to prometheus_client 0.24 (#19379)

Fixes #19375 

`prometheus_client` 0.24 makes `Collector` a generic type. 
Previously, `InFlightGauge` inherited from both `Generic[MetricsEntry]`
and `Collector`, resulting in the error `TypeError: cannot create a
consistent MRO` when using `prometheus_client` >= 0.24. This behaviour
of disallowing multiple `Generic` inheritance is more strictly enforced
starting with python 3.14, but can still lead to issues with earlier
versions of python.

This PR separates runtime and typing inheritance for `InFlightGauge`:
- Runtime: `InFlightGauge` inherits only from `Collector`
- Typing: `InFlightGauge` is generic

This preserves static typing, avoids MRO conflicts, and supports both
`prometheus_client` <0.24 and >=0.24.

I have tested these changes out locally with `prometheus_client` 0.23.1
& 0.24 on python 3.14 while sending a bunch of messages over federation
and watching a grafana dashboard configured to show
`synapse_util_metrics_block_in_flight_total` &
`synapse_util_metrics_block_in_flight_real_time_sum` (the only metric
setup to use `InFlightGauge`) and things are working in each case.
a1e9abc7df/synapse/util/metrics.py (L112-L119)

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scripts-dev/mypy_synapse_plugin.py

@@ -133,6 +133,7 @@ prometheus_metric_fullname_to_label_arg_map: Mapping[str, ArgLocation | None] =
     "prometheus_client.metrics.Info": ArgLocation("labelnames", 2),
     "prometheus_client.metrics.Enum": ArgLocation("labelnames", 2),
     "synapse.metrics.LaterGauge": ArgLocation("labelnames", 2),
+    "synapse.metrics._InFlightGaugeRuntime": ArgLocation("labels", 2),
     "synapse.metrics.InFlightGauge": ArgLocation("labels", 2),
     "synapse.metrics.GaugeBucketCollector": ArgLocation("labelnames", 2),
     "prometheus_client.registry.Collector": None,