Instrument _check_sigs_and_hash_and_fetch to trace time spent in child concurrent calls (#13588)

Instrument `_check_sigs_and_hash_and_fetch` to trace time spent in child concurrent calls because I've see `_check_sigs_and_hash_and_fetch` take [10.41s to process 100 events](https://github.com/matrix-org/synapse/issues/13587)

Fix https://github.com/matrix-org/synapse/issues/13587

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 SynapseTags, set_tag, tag_args, trace
+from synapse.logging.opentracing import SynapseTags, log_kv, 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
@@ -587,11 +587,15 @@ class FederationClient(FederationBase):
         Returns:
             A list of PDUs that have valid signatures and hashes.
         """
+        set_tag(
+            SynapseTags.RESULT_PREFIX + "pdus.length",
+            str(len(pdus)),
+        )
 
         # We limit how many PDUs we check at once, as if we try to do hundreds
         # of thousands of PDUs at once we see large memory spikes.
 
-        valid_pdus = []
+        valid_pdus: List[EventBase] = []
 
         async def _execute(pdu: EventBase) -> None:
             valid_pdu = await self._check_sigs_and_hash_and_fetch_one(
@@ -607,6 +611,8 @@ class FederationClient(FederationBase):
 
         return valid_pdus
 
+    @trace
+    @tag_args
     async def _check_sigs_and_hash_and_fetch_one(
         self,
         pdu: EventBase,
@@ -639,16 +645,27 @@ class FederationClient(FederationBase):
         except InvalidEventSignatureError as e:
             logger.warning(
                 "Signature on retrieved event %s was invalid (%s). "
-                "Checking local store/orgin server",
+                "Checking local store/origin server",
                 pdu.event_id,
                 e,
             )
+            log_kv(
+                {
+                    "message": "Signature on retrieved event was invalid. "
+                    "Checking local store/origin server",
+                    "event_id": pdu.event_id,
+                    "InvalidEventSignatureError": e,
+                }
+            )
 
         # Check local db.
         res = await self.store.get_event(
             pdu.event_id, allow_rejected=True, allow_none=True
         )
 
+        # If the PDU fails its signature check and we don't have it in our
+        # database, we then request it from sender's server (if that is not the
+        # same as `origin`).
         pdu_origin = get_domain_from_id(pdu.sender)
         if not res and pdu_origin != origin:
             try: