Various opentracing enhancements (#11619)

* Wrap `auth.get_user_by_req` in an opentracing span

give `get_user_by_req` its own opentracing span, since it can result in a
non-trivial number of sub-spans which it is useful to group together.

This requires a bit of reorganisation because it also sets some tags (and may
force tracing) on the servlet span.

* Emit opentracing span for encoding json responses

This can be a significant time sink.

* Rename all sync spans with a prefix

* Write an opentracing span for encoding sync response

* opentracing span to group generate_room_entries

* opentracing spans within sync.encode_response

* changelog

* Use the `trace` decorator instead of context managers

synapse/rest/client/sync.py

@@ -48,6 +48,7 @@ from synapse.handlers.sync import (
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_boolean, parse_integer, parse_string
 from synapse.http.site import SynapseRequest
+from synapse.logging.opentracing import trace
 from synapse.types import JsonDict, StreamToken
 from synapse.util import json_decoder
 
@@ -222,6 +223,7 @@ class SyncRestServlet(RestServlet):
         logger.debug("Event formatting complete")
         return 200, response_content
 
+    @trace(opname="sync.encode_response")
     async def encode_response(
         self,
         time_now: int,
@@ -332,6 +334,7 @@ class SyncRestServlet(RestServlet):
             ]
         }
 
+    @trace(opname="sync.encode_joined")
     async def encode_joined(
         self,
         rooms: List[JoinedSyncResult],
@@ -368,6 +371,7 @@ class SyncRestServlet(RestServlet):
 
         return joined
 
+    @trace(opname="sync.encode_invited")
     async def encode_invited(
         self,
         rooms: List[InvitedSyncResult],
@@ -406,6 +410,7 @@ class SyncRestServlet(RestServlet):
 
         return invited
 
+    @trace(opname="sync.encode_knocked")
     async def encode_knocked(
         self,
         rooms: List[KnockedSyncResult],
@@ -460,6 +465,7 @@ class SyncRestServlet(RestServlet):
 
         return knocked
 
+    @trace(opname="sync.encode_archived")
     async def encode_archived(
         self,
         rooms: List[ArchivedSyncResult],