Refactor LaterGauge metrics to be homeserver-scoped

[MadLittleMods]

Refactor LaterGauge metrics to be homeserver-scoped

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 TODO metrics with the server_name label

Todo

• Add lint -> Update metrics linting to be able to handle custom metrics #18733
• Find solution for global LaterGauge instances
  • synapse/http/request_metrics.py
  • synapse/replication/tcp/protocol.py
  • synapse/util/ratelimitutils.py
• Wait for Refactor Counter metrics to be homeserver-scoped #18656 to merge

Dev notes

Global LaterGauge instances:

• synapse/http/request_metrics.py
• synapse/replication/tcp/protocol.py
• synapse/util/ratelimitutils.py

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)

[devonh]

Again - straightforward changes following the same metrics patterns as other PRs.
Looks good 👍

...Assuming you can make CI happy of course.

[MadLittleMods]

Thanks for the review @devonh 🐓