Devon Hudson 8b36740bad 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.
https://github.com/element-hq/synapse/blob/a1e9abc7df3e6c43a95cba059348546a4c9d4491/synapse/util/metrics.py#L112-L119

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2026-01-16 20:35:30 +00:00

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