Fix Failed to stop metrics warnings in request metrics

[MadLittleMods]

Fix Failed to stop metrics warnings in request metrics

Failed to stop metrics: TypeError("prometheus_client.metrics.MetricWrapperBase.labels() got multiple values for keyword argument 'server_name'")

Noticed while running and debugging some tests.

This bug was introduced in #18724

Testing strategy

1. SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.handlers.test_message
2. Inspect _trial_temp/test.log to see any warnings/errors in the logs (search for Failed to stop metrics)

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]

Same changelog as #18724 so they merge

[MadLittleMods]

Thanks for the review @devonh 🦘