Don't update event cache hit ratio from get_joined_users

Otherwise the hit ration of plain get_events gets completely skewed by
calls to get_joined_users* functions.

synapse/storage/roommember.py

@@ -421,9 +421,13 @@ class RoomMemberStore(SQLBaseStore):
         # We check if we have any of the member event ids in the event cache
         # before we ask the DB
 
+        # We don't update the event cache hit ratio as it completely throws off
+        # the hit ratio counts. After all, we don't populate the cache if we
+        # miss it here
         event_map = self._get_events_from_cache(
             member_event_ids,
             allow_rejected=False,
+            update_metrics=False,
         )
 
         missing_member_event_ids = []