Fix `get_metadata_for_events` (#12904)

This method was introduced in #12852. It is using the `state_key` column from
the `events` table, which is not (yet) reliable (see #11496).

changelog.d/12904.misc

@@ -0,0 +1 @@
+Pull out less state when handling gaps in room DAG.

synapse/storage/databases/main/state.py

@@ -167,8 +167,8 @@ class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
             )
 
             sql = f"""
-                SELECT e.event_id, e.room_id, e.type, e.state_key FROM events AS e
-                LEFT JOIN state_events USING (event_id)
+                SELECT e.event_id, e.room_id, e.type, se.state_key FROM events AS e
+                LEFT JOIN state_events se USING (event_id)
                 WHERE {clause}
             """