Add debug logging for issue #9533 (#9959)

Hopefully this will help us track down where to-device messages are getting
lost/delayed.

synapse/federation/sender/per_destination_queue.py

@@ -28,6 +28,7 @@ from synapse.api.presence import UserPresenceState
 from synapse.events import EventBase
 from synapse.federation.units import Edu
 from synapse.handlers.presence import format_user_presence_state
+from synapse.logging import issue9533_logger
 from synapse.logging.opentracing import SynapseTags, set_tag
 from synapse.metrics import sent_transactions_counter
 from synapse.metrics.background_process_metrics import run_as_background_process
@@ -574,6 +575,14 @@ class PerDestinationQueue:
             for content in contents
         ]
 
+        if edus:
+            issue9533_logger.debug(
+                "Sending %i to-device messages to %s, up to stream id %i",
+                len(edus),
+                self._destination,
+                stream_id,
+            )
+
         return (edus, stream_id)
 
     def _start_catching_up(self) -> None: