Debug for device lists updates (#11760)

Debug for #8631.

I'm having a hard time tracking down what's going wrong in that issue.
In the reported example, I could see server A sending federation traffic
to server B and all was well. Yet B reports out-of-sync device updates
from A.

I couldn't see what was _in_ the events being sent from A to B. So I
have added some crude logging to track

- when we have updates to send to a remote HS
- the edus we actually accumulate to send
- when a federation transaction includes a device list update edu
- when such an EDU is received

This is a bit of a sledgehammer.

synapse/storage/databases/main/devices.py

@@ -53,6 +53,7 @@ if TYPE_CHECKING:
     from synapse.server import HomeServer
 
 logger = logging.getLogger(__name__)
+issue_8631_logger = logging.getLogger("synapse.8631_debug")
 
 DROP_DEVICE_LIST_STREAMS_NON_UNIQUE_INDEXES = (
     "drop_device_list_streams_non_unique_indexes"
@@ -229,6 +230,12 @@ class DeviceWorkerStore(SQLBaseStore):
         if not updates:
             return now_stream_id, []
 
+        if issue_8631_logger.isEnabledFor(logging.DEBUG):
+            data = {(user, device): stream_id for user, device, stream_id, _ in updates}
+            issue_8631_logger.debug(
+                "device updates need to be sent to %s: %s", destination, data
+            )
+
         # get the cross-signing keys of the users in the list, so that we can
         # determine which of the device changes were cross-signing keys
         users = {r[0] for r in updates}
@@ -365,6 +372,17 @@ class DeviceWorkerStore(SQLBaseStore):
             #        and remove the length budgeting above.
             results.append(("org.matrix.signing_key_update", result))
 
+        if issue_8631_logger.isEnabledFor(logging.DEBUG):
+            for (user_id, edu) in results:
+                issue_8631_logger.debug(
+                    "device update to %s for %s from %s to %s: %s",
+                    destination,
+                    user_id,
+                    from_stream_id,
+                    last_processed_stream_id,
+                    edu,
+                )
+
         return last_processed_stream_id, results
 
     def _get_device_updates_by_remote_txn(