Instrument `get_metadata_for_events` for tracing (#13730)

When backfilling, `_get_state_ids_after_missing_prev_event` calls [`get_metadata_for_events`](https://github.com/matrix-org/synapse/blob/26bc26586b4b95d63ce7e453e9312469843f796e/synapse/handlers/federation_event.py#L1133). For `#matrix:matrix.org`, it's called with 77k `state_events` which means 77 calls to the database and takes 28 seconds.

changelog.d/13730.misc

+Instrument `get_metadata_for_events` for understandable traces in Jaeger.

synapse/storage/databases/main/state.py

@@ -23,6 +23,7 @@ from synapse.api.errors import NotFoundError, UnsupportedRoomVersionError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
+from synapse.logging.opentracing import trace
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.database import (
     DatabasePool,
@@ -143,6 +144,7 @@ class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
 
         return room_version
 
+    @trace
     async def get_metadata_for_events(
         self, event_ids: Collection[str]
     ) -> Dict[str, EventMetadata]: