Cleanup opentracing logging for syncs (#10828)

We added a bunch of spans in #10704, but this ended up adding a lot of
redundant spans for rooms where nothing changed, so instead we only
start the span if there might be something interesting going on.

changelog.d/10828.bugfix

@@ -0,0 +1 @@
+Added opentrace logging to help debug #9424.

synapse/handlers/sync.py

@@ -1533,9 +1533,6 @@ class SyncHandler:
         newly_left_rooms = room_changes.newly_left_rooms
 
         async def handle_room_entries(room_entry: "RoomSyncResultBuilder"):
-            with start_active_span("generate_room_entry"):
-                set_tag("room_id", room_entry.room_id)
-                log_kv({"events": len(room_entry.events or [])})
             logger.debug("Generating room entry for %s", room_entry.room_id)
             res = await self._generate_room_entry(
                 sync_result_builder,
@@ -1960,6 +1957,11 @@ class SyncHandler:
         room_id = room_builder.room_id
         since_token = room_builder.since_token
         upto_token = room_builder.upto_token
+
+        with start_active_span("generate_room_entry"):
+            set_tag("room_id", room_id)
+            log_kv({"events": len(events or ())})
+
             log_kv(
                 {
                     "since_token": since_token,
@@ -2013,21 +2015,34 @@ class SyncHandler:
                 account_data_events.append({"type": "m.tag", "content": {"tags": tags}})
 
             for account_data_type, content in account_data.items():
-            account_data_events.append({"type": account_data_type, "content": content})
+                account_data_events.append(
+                    {"type": account_data_type, "content": content}
+                )
 
-        account_data_events = sync_config.filter_collection.filter_room_account_data(
+            account_data_events = (
+                sync_config.filter_collection.filter_room_account_data(
                     account_data_events
                 )
+            )
 
             ephemeral = sync_config.filter_collection.filter_room_ephemeral(ephemeral)
 
             if not (
-            always_include or batch or account_data_events or ephemeral or full_state
+                always_include
+                or batch
+                or account_data_events
+                or ephemeral
+                or full_state
             ):
                 return
 
             state = await self.compute_state_delta(
-            room_id, batch, sync_config, since_token, now_token, full_state=full_state
+                room_id,
+                batch,
+                sync_config,
+                since_token,
+                now_token,
+                full_state=full_state,
             )
 
             summary: Optional[JsonDict] = {}