Fix spurious warning when fetching state after a missing prev event (#13258)

changelog.d/13258.misc

@@ -0,0 +1 @@
+Fix spurious warning when fetching state after a missing prev event.

synapse/handlers/federation_event.py

@@ -1037,6 +1037,9 @@ class FederationEventHandler:
         # XXX: this doesn't sound right? it means that we'll end up with incomplete
         #   state.
         failed_to_fetch = desired_events - event_metadata.keys()
+        # `event_id` could be missing from `event_metadata` because it's not necessarily
+        # a state event. We've already checked that we've fetched it above.
+        failed_to_fetch.discard(event_id)
         if failed_to_fetch:
             logger.warning(
                 "Failed to fetch missing state events for %s %s",