Run `_handle_queued_pdus` as a background process (#12041)

... to ensure it gets a proper log context, mostly.

changelog.d/12041.misc

@@ -0,0 +1 @@
+After joining a room, create a dedicated logcontext to process the queued events.

synapse/handlers/federation.py

@@ -49,8 +49,8 @@ from synapse.logging.context import (
     make_deferred_yieldable,
     nested_logging_context,
     preserve_fn,
-    run_in_background,
 )
+from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.replication.http.federation import (
     ReplicationCleanRoomRestServlet,
     ReplicationStoreRoomOnOutlierMembershipRestServlet,
@@ -559,7 +559,9 @@ class FederationHandler:
             # lots of requests for missing prev_events which we do actually
             # have. Hence we fire off the background task, but don't wait for it.
 
-            run_in_background(self._handle_queued_pdus, room_queue)
+            run_as_background_process(
+                "handle_queued_pdus", self._handle_queued_pdus, room_queue
+            )
 
     async def do_knock(
         self,