Tag trace with instance name (#13761)

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.

As suggested by @clokep and @reivilibre in,

 - https://github.com/matrix-org/synapse/pull/13729#discussion_r964719258
 - https://github.com/matrix-org/synapse/pull/13729#discussion_r964733578

2 files changed, 11 insertions, 2 deletions

diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 8e54ef84b2..4a75eb6b21 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -32,6 +32,7 @@ from synapse.appservice import ApplicationService
 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 @@ class Auth:
                 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)
diff --git a/synapse/logging/opentracing.py b/synapse/logging/opentracing.py
index adf3f54770..ca2735dd6d 100644
--- a/synapse/logging/opentracing.py
+++ b/synapse/logging/opentracing.py
@@ -298,6 +298,8 @@ 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"
 
@@ -1043,11 +1045,11 @@ 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)