Add Measure block for persist_events

This seems like a useful thing to measure.

synapse/storage/events.py

@@ -283,6 +283,7 @@ class EventsStore(EventsWorkerStore):
     def _maybe_start_persisting(self, room_id):
         @defer.inlineCallbacks
         def persisting_queue(item):
+            with Measure(self._clock, "persist_events"):
                 yield self._persist_events(
                     item.events_and_contexts,
                     backfilled=item.backfilled,