Label Tempo's exemplars trace_id so they link to the trace

Tempo's metrics-generator defaults the exemplar label to traceID, which
the Prometheus datasource's exemplarTraceIdDestinations (trace_id) never
matched: exemplars rendered in Grafana but didn't link to Tempo. Set
trace_id_label_name in the overrides so trace_id is the one correlation
key everywhere.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

k8s/README.md

@@ -45,6 +45,10 @@ OTLP/gRPC (4317) is cluster-internal only. Apps use OTLP/HTTP.
   Traefik routes to that container's `:9464` behind basic auth. The scrape
   credentials are the `prometheus-scrape-auth` secret, mounted at
   `/etc/prometheus/secrets`.
+- **Tempo labels exemplars `trace_id`** (`trace_id_label_name` in the
+  overrides). Tempo's default is `traceID`, which the Prometheus datasource's
+  `exemplarTraceIdDestinations` (`trace_id`) doesn't match, so span-metric
+  exemplars rendered in Grafana but didn't link to the trace.
 
 The CLAUDE.md platform facts still hold:
 - OTLP metrics and logs are rejected: Alloy returns a 404 on `/v1/metrics` and

k8s/tempo/tempo.yml

@@ -45,3 +45,8 @@ overrides:
   defaults:
     metrics_generator:
       processors: [service-graphs, span-metrics, local-blocks]
+      # Not in the Ansible template. Without it the generator labels exemplars
+      # traceID (the code default; the docs wrongly say trace_id), which the
+      # Prometheus datasource's exemplarTraceIdDestinations (trace_id) never
+      # matches, so exemplars render but don't link to Tempo.
+      trace_id_label_name: trace_id