Tag trace with instance name

As suggested in:

 - #13729 (comment)
 - #13729 (comment)

synapse/api/auth.py

@@ -32,6 +32,7 @@
 from synapse.http import get_request_user_agent
 from synapse.http.site import SynapseRequest
 from synapse.logging.opentracing import (
+    SynapseTags,
     active_span,
     force_tracing,
     start_active_span,
@@ -161,6 +162,12 @@ async def get_user_by_req(
                 parent_span.set_tag(
                     "authenticated_entity", requester.authenticated_entity
                 )
+                # We tag the Synapse instance name so that it's an easy jumping
+                # off point into the logs. Can also be used to filter for an
+                # instance that is under load.
+                parent_span.set_tag(
+                    SynapseTags.INSTANCE_NAME, self.hs.get_instance_name()
+                )
                 parent_span.set_tag("user_id", requester.user.to_string())
                 if requester.device_id is not None:
                     parent_span.set_tag("device_id", requester.device_id)

synapse/logging/opentracing.py

@@ -295,6 +295,8 @@ class SynapseTags:
     # Whether the sync response has new data to be returned to the client.
     SYNC_RESULT = "sync.new_data"
 
+    INSTANCE_NAME = "instance_name"
+
     # incoming HTTP request ID  (as written in the logs)
     REQUEST_ID = "request_id"
 
@@ -1032,11 +1034,11 @@ def trace_servlet(
             # with JsonResource).
             scope.span.set_operation_name(request.request_metrics.name)
 
-            # set the tags *after* the servlet completes, in case it decided to
-            # prioritise the span (tags will get dropped on unprioritised spans)
             request_tags[
                 SynapseTags.REQUEST_TAG
             ] = request.request_metrics.start_context.tag
 
+            # set the tags *after* the servlet completes, in case it decided to
+            # prioritise the span (tags will get dropped on unprioritised spans)
             for k, v in request_tags.items():
                 scope.span.set_tag(k, v)