Improve opentracing for incoming HTTP requests (#11618)

* remove `start_active_span_from_request`

Instead, pull out a separate function, `span_context_from_request`, to extract
the parent span, which we can then pass into `start_active_span` as
normal. This seems to be clearer all round.

* Remove redundant tags from `incoming-federation-request`

These are all wrapped up inside a parent span generated in AsyncResource, so
there's no point duplicating all the tags that are set there.

* Leave request spans open until the request completes

It may take some time for the response to be encoded into JSON, and that JSON
to be streamed back to the client, and really we want that inside the top-level
span, so let's hand responsibility for closure to the SynapseRequest.

* opentracing logs for HTTP request events

* changelog

synapse/federation/transport/server/_base.py

@@ -22,13 +22,11 @@ from synapse.api.urls import FEDERATION_V1_PREFIX
 from synapse.http.server import HttpServer, ServletCallback
 from synapse.http.servlet import parse_json_object_from_request
 from synapse.http.site import SynapseRequest
-from synapse.logging import opentracing
 from synapse.logging.context import run_in_background
 from synapse.logging.opentracing import (
-    SynapseTags,
-    start_active_span,
-    start_active_span_from_request,
-    tags,
+    set_tag,
+    span_context_from_request,
+    start_active_span_follows_from,
     whitelisted_homeserver,
 )
 from synapse.server import HomeServer
@@ -279,30 +277,19 @@ class BaseFederationServlet:
                 logger.warning("authenticate_request failed: %s", e)
                 raise
 
-            request_tags = {
-                SynapseTags.REQUEST_ID: request.get_request_id(),
-                tags.SPAN_KIND: tags.SPAN_KIND_RPC_SERVER,
-                tags.HTTP_METHOD: request.get_method(),
-                tags.HTTP_URL: request.get_redacted_uri(),
-                tags.PEER_HOST_IPV6: request.getClientIP(),
-                "authenticated_entity": origin,
-                "servlet_name": request.request_metrics.name,
-            }
+            # update the active opentracing span with the authenticated entity
+            set_tag("authenticated_entity", origin)
 
-            # Only accept the span context if the origin is authenticated
-            # and whitelisted
+            # if the origin is authenticated and whitelisted, link to its span context
+            context = None
             if origin and whitelisted_homeserver(origin):
-                scope = start_active_span_from_request(
-                    request, "incoming-federation-request", tags=request_tags
-                )
-            else:
-                scope = start_active_span(
-                    "incoming-federation-request", tags=request_tags
-                )
+                context = span_context_from_request(request)
+
+            scope = start_active_span_follows_from(
+                "incoming-federation-request", contexts=(context,) if context else ()
+            )
 
             with scope:
-                opentracing.inject_response_headers(request.responseHeaders)
-
                 if origin and self.RATELIMIT:
                     with ratelimiter.ratelimit(origin) as d:
                         await d