Update TODO list of conflicting areas where we encounter metrics being clobbered (ApplicationService) (#19040)

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

```
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
```

Author: MadLittleMods

changelog.d/19040.misc

@@ -0,0 +1 @@
+Update TODO list of conflicting areas where we encounter metrics being clobbered (`ApplicationService`).

synapse/util/metrics.py

@@ -323,9 +323,13 @@ def register_hook(
         if server_hooks.get(metric_name) is not None:
             # TODO: This should be an `assert` since registering the same metric name
             # multiple times will clobber the old metric.
-            # We currently rely on this behaviour as we instantiate multiple
-            # `SyncRestServlet`, one per listener, and in the `__init__` we setup a new
-            # LruCache.
+            #
+            # We currently rely on this behaviour in a few places:
+            #  - We instantiate multiple `SyncRestServlet`, one per listener, and in the
+            #   `__init__` we setup a new `LruCache`.
+            #  - We instantiate multiple `ApplicationService` (one per configured
+            #    application service) which use the `@cached` decorator on some methods.
+            #
             # Once the above behaviour is changed, this should be changed to an `assert`.
             logger.error(
                 "Metric named %s already registered for server %s",