Log when we delete room in bg update (#6816)

changelog.d/6816.misc

@@ -0,0 +1 @@
+Add background update to clean out left rooms from current state.

synapse/storage/data_stores/main/state.py

@@ -365,6 +365,8 @@ class MainStateBackgroundUpdateStore(RoomMemberWorkerStore):
 
             left_rooms = set(room_ids) - joined_room_ids
 
+            logger.info("Deleting current state left rooms: %r", left_rooms)
+
             # First we get all users that we still think were joined to the
             # room. This is so that we can mark those device lists as
             # potentially stale, since there may have been a period where the