Sliding Sync: Add more tracing (#17514)

Spawning from looking at a couple traces and wanting a little more info.

Follow-up to github.com/element-hq/synapse/pull/17501

The changes in this PR allow you to find slow Sliding Sync traces ignoring the
`wait_for_events` time. In Jaeger, you can now filter for the `current_sync_for_user`
operation with `RESULT.result=true` indicating that it actually returned non-empty results.

If you want to find traces for your own user, you can use
`RESULT.result=true ARG.sync_config.user="@madlittlemods:matrix.org"`

1 files changed, 9 insertions, 0 deletions

diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py

index 4f2c552af2..8c5db2a513 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -899,6 +899,9 @@ class SlidingSyncRestServlet(RestServlet):
         body = parse_and_validate_json_object_from_request(request, SlidingSyncBody)
 
         # Tag and log useful data to differentiate requests.
+        set_tag(
+            "sliding_sync.sync_type", "initial" if from_token is None else "incremental"
+        )
         set_tag("sliding_sync.conn_id", body.conn_id or "")
         log_kv(
             {
@@ -912,6 +915,12 @@ class SlidingSyncRestServlet(RestServlet):
                 "sliding_sync.room_subscriptions": list(
                     (body.room_subscriptions or {}).keys()
                 ),
+                # We also include the number of room subscriptions because logs are
+                # limited to 1024 characters and the large room ID list above can be cut
+                # off.
+                "sliding_sync.num_room_subscriptions": len(
+                    (body.room_subscriptions or {}).keys()
+                ),
             }
         )