Instrument the federation/backfill part of /messages (#13489)

Instrument the federation/backfill part of `/messages` so it's easier to follow what's going on in Jaeger when viewing a trace.

Split out from https://github.com/matrix-org/synapse/pull/13440

Follow-up from https://github.com/matrix-org/synapse/pull/13368

Part of https://github.com/matrix-org/synapse/issues/13356

synapse/federation/federation_client.py

@@ -61,7 +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.logging.opentracing import SynapseTags, set_tag, tag_args, 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
@@ -235,6 +235,7 @@ class FederationClient(FederationBase):
         )
 
     @trace
+    @tag_args
     async def backfill(
         self, dest: str, room_id: str, limit: int, extremities: Collection[str]
     ) -> Optional[List[EventBase]]:
@@ -337,6 +338,8 @@ class FederationClient(FederationBase):
 
         return None
 
+    @trace
+    @tag_args
     async def get_pdu(
         self,
         destinations: Iterable[str],
@@ -448,6 +451,8 @@ class FederationClient(FederationBase):
 
         return event_copy
 
+    @trace
+    @tag_args
     async def get_room_state_ids(
         self, destination: str, room_id: str, event_id: str
     ) -> Tuple[List[str], List[str]]:
@@ -467,6 +472,23 @@ class FederationClient(FederationBase):
         state_event_ids = result["pdu_ids"]
         auth_event_ids = result.get("auth_chain_ids", [])
 
+        set_tag(
+            SynapseTags.RESULT_PREFIX + "state_event_ids",
+            str(state_event_ids),
+        )
+        set_tag(
+            SynapseTags.RESULT_PREFIX + "state_event_ids.length",
+            str(len(state_event_ids)),
+        )
+        set_tag(
+            SynapseTags.RESULT_PREFIX + "auth_event_ids",
+            str(auth_event_ids),
+        )
+        set_tag(
+            SynapseTags.RESULT_PREFIX + "auth_event_ids.length",
+            str(len(auth_event_ids)),
+        )
+
         if not isinstance(state_event_ids, list) or not isinstance(
             auth_event_ids, list
         ):
@@ -474,6 +496,8 @@ class FederationClient(FederationBase):
 
         return state_event_ids, auth_event_ids
 
+    @trace
+    @tag_args
     async def get_room_state(
         self,
         destination: str,
@@ -533,6 +557,7 @@ class FederationClient(FederationBase):
 
         return valid_state_events, valid_auth_events
 
+    @trace
     async def _check_sigs_and_hash_and_fetch(
         self,
         origin: str,