Add CPU metrics for _fetch_event_list

add a Measure block on _fetch_event_list, in the hope that we can better
measure CPU usage here.

synapse/storage/events_worker.py

@@ -222,10 +222,8 @@ class EventsWorkerStore(SQLBaseStore):
         """Takes a database connection and waits for requests for events from
         the _event_fetch_list queue.
         """
-        event_list = []
         i = 0
         while True:
-            try:
             with self._event_fetch_lock:
                 event_list = self._event_fetch_list
                 self._event_fetch_list = []
@@ -241,6 +239,22 @@ class EventsWorkerStore(SQLBaseStore):
                         continue
                 i = 0
 
+            self._fetch_event_list(conn, event_list)
+
+    def _fetch_event_list(self, conn, event_list):
+        """Handle a load of requests from the _event_fetch_list queue
+
+        Args:
+            conn (twisted.enterprise.adbapi.Connection): database connection
+
+            event_list (list[Tuple[list[str], Deferred]]):
+                The fetch requests. Each entry consists of a list of event
+                ids to be fetched, and a deferred to be completed once the
+                events have been fetched.
+
+        """
+        with Measure(self._clock, "_fetch_event_list"):
+            try:
                 event_id_lists = zip(*event_list)[0]
                 event_ids = [
                     item for sublist in event_id_lists for item in sublist
@@ -280,7 +294,6 @@ class EventsWorkerStore(SQLBaseStore):
                             with PreserveLoggingContext():
                                 d.errback(e)
 
-                if event_list:
                 with PreserveLoggingContext():
                     self.hs.get_reactor().callFromThread(fire, event_list)