Update TODO list of conflicting areas where we encounter metrics being clobbered (ApplicationService)

[MadLittleMods]

Update TODO list of conflicting areas where we encounter metrics being clobbered (ApplicationService)

As encountered by @wrjlewis,

2025-10-10 15:30:00,026 - synapse.util.metrics - 330 - ERROR - notify_interested_services-0 - Metric named cache_lru_cache__matches_user_in_member_list_example.com already registered for server example.com
	2025-10-10 16:30:00.167
2025-10-10 15:30:00,026 - synapse.util.metrics - 330 - ERROR - notify_interested_services-0 - Metric named cache_lru_cache_is_interested_in_room_example.com already registered for server example.com
	2025-10-10 16:30:00.167
2025-10-10 15:30:00,025 - synapse.util.metrics - 330 - ERROR - notify_interested_services-0 - Metric named cache_lru_cache_is_interested_in_event_example.com already registered for server example.com
	2025-10-10 16:29:15.560
2025-10-10 15:29:15,449 - synapse.util.metrics - 330 - ERROR - notify_interested_services_ephemeral-0 - Metric named cache_lru_cache__matches_user_in_member_list_example.com already registered for server example.com
	2025-10-10 16:29:15.560
2025-10-10 15:29:15,449 - synapse.util.metrics - 330 - ERROR - notify_interested_services_ephemeral-0 - Metric named cache_lru_cache_is_interested_in_room_example.com already registered for server example.com

These errors are harmless and are a long-standing issue that is just now being logged, see #19042

Dev notes

• Refactor metrics to be scoped to the homeserver #18592
• Cleanly shutdown SynapseHomeServer object #18828
• Update metrics linting to be able to handle custom metrics #18733
• Refactor cache metrics to be homeserver-scoped #18604

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]

Tracking issue: #19042