Instrument `/messages` for understandable traces in Jaeger (#13368)

In Jaeger:

 - Before: huge list of uncategorized database calls
 - After: nice and collapsible into units of work

1 files changed, 2 insertions, 0 deletions

diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py

index 6a8d76529b..54ffbd8170 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -61,6 +61,7 @@ from synapse.federation.federation_base import (
 )
 from synapse.federation.transport.client import SendJoinResponse
 from synapse.http.types import QueryParams
+from synapse.logging.opentracing import trace
 from synapse.types import JsonDict, UserID, get_domain_from_id
 from synapse.util.async_helpers import concurrently_execute
 from synapse.util.caches.expiringcache import ExpiringCache
@@ -233,6 +234,7 @@ class FederationClient(FederationBase):
             destination, content, timeout
         )
 
+    @trace
     async def backfill(
         self, dest: str, room_id: str, limit: int, extremities: Collection[str]
     ) -> Optional[List[EventBase]]: