Trace how many new events from the backfill response we need to process

changelog.d/15633.misc

@@ -0,0 +1 @@
+Trace how many new events from the backfill response we need to process.

synapse/handlers/federation_event.py

@@ -890,6 +890,11 @@ async def _process_pulled_events(
             # Continue on with the events that are new to us.
             new_events.append(event)
 
+        set_tag(
+            SynapseTags.RESULT_PREFIX + "new_events.length",
+            str(len(new_events)),
+        )
+
         # We want to sort these by depth so we process them and
         # tell clients about them in order.
         sorted_events = sorted(new_events, key=lambda x: x.depth)