Fix InFlightGauge typing to allow upgrading to prometheus_client 0.24

[devonh]

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.

synapse/synapse/util/metrics.py

Lines 112 to 119 in a1e9abc

	in_flight: InFlightGauge[_BlockInFlightMetric] = InFlightGauge(
	"synapse_util_metrics_block_in_flight",
	desc="Tracks the number of blocks currently active",
	labels=["block_name", SERVER_NAME_LABEL],
	# Matches the fields in the `_BlockInFlightMetric`
	sub_metrics=["real_time_max", "real_time_sum"],
	)
	"""Tracks the number of blocks currently active"""

Pull Request Checklist

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[spaetz]

Just adding for the record that this is not a python 3.14 issue. Synapse refuses to start with the same error on a python 3.13 venv and prometheus_client 0.24. So the amount of people that might be affected could be somewhat larger.

[anoadragon453]

LGTM, just a couple small changes. Thanks for taking this one on!