Use homeserver specific Prometheus CollectorRegistry for metrics instead of global REGISTRY

[MadLittleMods]

Use homeserver specific Prometheus CollectorRegistry (hs.metrics_collector_registry) for metrics instead of global REGISTRY which can be polluted with whatever other code/libraries that are running in the same Python process that also use the default REGISTRY.

This is specifically spawning from running multiple Synapse in the same Python process and wanting separate metrics.

Other breaking changes:

• Remove the specialized metrics listener in favor of our standard http listener with the metrics resource

Initial exploration for #18592

Todo

Update: This is now better tracked by #18592

• Ensure _set_prometheus_client_use_created_metrics is used with the new path (PrometheusMetricsHackTestCase)
• Refactor built-in Prometheus metrics to use registry=hs.metrics_collector_registry
  • Counter
  • Histogram
  • Gauge
  • Summary
  • Info
  • Enum
• Refactor our custom Prometheus Collector metrics to use registry=hs.metrics_collector_registry
  • LaterGauge
  • GaugeBucketCollector
  • CPUMetrics
  • GCCounts
  • PyPyGCStats
  • ReactorLastSeenMetric
  • Background process _Collector
  • JemallocCollector
  • DynamicCollectorRegistry

---

• LruCache
  • @cached
    • Make sure _CachedFunctionDescriptor is accounted for in the mypy plugin (scripts-dev/mypy_synapse_plugin.py)

Dev notes

Parts of this PR have been split out to other PRs:

• Remove metrics listener type in favor of http listener with metrics resource #18584

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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  • 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.
• Code style is correct (run the linters)

[MadLittleMods]

Closing in favor of the smaller PRs that target #18592