Refactor Counter metrics to be homeserver-scoped

[MadLittleMods]

Bulk refactor Counter metrics to be homeserver-scoped. We also add lints to make sure that new Counter metrics don't sneak in without using the server_name label (SERVER_NAME_LABEL).

All of the "Fill in" commits are just bulk refactor.

Part of #18592

Testing strategy

1. Add the metrics listener in your homeserver.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']

2. Start the homeserver: poetry run synapse_homeserver --config-path homeserver.yaml
3. Fetch http://localhost:9322/_synapse/metrics and/or http://localhost:9323/metrics
4. Observe response includes the synapse_user_registrations_total, synapse_http_server_response_count_total, etc metrics with the server_name label

Dev notes

mypy plugins

https://mypy.readthedocs.io/en/latest/extending_mypy.html

poetry run mypy synapse/util/ratelimitutils.py

---

ErrorCodes defined in mypy plugins seemingly cannot be ignored by --disable-error-code python/mypy#12987

(doesn't work because of above issue)

poetry run mypy --disable-error-code missing-server-name-label
...
mypy: error: Invalid error code(s): missing-server-name-label

Refactor notes

More special refactors:

• synapse/http/request_metrics.py
• synapse/metrics/background_process_metrics.py

Unused metrics:

• notified_events_counter
• push_rules_invalidation_counter
• push_rules_state_size_counter
• remove_pusher_counter

Todo

• Add lint for metric instantiation
• Add lint for metric.labels(...)
  • No need as if you pass in the wrong number of labels, then prometheus_client will raise a ValueError at runtime. Ideally, we'd be able to catch this at type-checking time but this is good enough.
• Wait for Refactor Measure block metrics to be homeserver-scoped (v2) #18601 to merge
• Wait for Refactor background process metrics to be homeserver-scoped #18670 to merge

Pull Request Checklist

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

[MadLittleMods]

New linting to prevent new Counter metrics from being introduced without the SERVER_NAME_LABEL

[devonh]

These changes all look good and are pretty straightforward swaps to add the new server name label.

Thanks for taking the time to cleanup things like docstrings along the way :)

[devonh]

Huh, odd. Your cleanup looks good to me 🤷

[MadLittleMods]

Thanks for the review @devonh 🐎