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/handlers/pagination.py b/synapse/handlers/pagination.py

index 6262a35822..e1e34e3b16 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -24,6 +24,7 @@ from synapse.api.errors import SynapseError
 from synapse.api.filtering import Filter
 from synapse.events.utils import SerializeEventConfig
 from synapse.handlers.room import ShutdownRoomResponse
+from synapse.logging.opentracing import trace
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage.state import StateFilter
 from synapse.streams.config import PaginationConfig
@@ -416,6 +417,7 @@ class PaginationHandler:
 
             await self._storage_controllers.purge_events.purge_room(room_id)
 
+    @trace
     async def get_messages(
         self,
         requester: Requester,