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.
matrix-org · Sep 7, 2022 · d4d3249 · d4d3249
1 parent 8d7fcf9
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diff --git a/changelog.d/13730.misc b/changelog.d/13730.misc
@@ -0,0 +1 @@
+Instrument `get_metadata_for_events` for understandable traces in Jaeger.
diff --git a/synapse/storage/databases/main/state.py b/synapse/storage/databases/main/state.py
@@ -23,6 +23,7 @@
 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 @@ def get_room_version_id_txn(self, txn: LoggingTransaction, room_id: str) -> str:
 
         return room_version
 
+    @trace
     async def get_metadata_for_events(
         self, event_ids: Collection[str]
     ) -> Dict[str, EventMetadata]: