Set explicit uid on Prometheus datasource for dashboard compatibility

Dashboard template variables reference datasource uid "prometheus" but
the provisioning config had no uid set, causing Grafana to auto-assign
a random one and break dashboard panel queries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/telemetry/grafana/provisioning/datasources/prometheus.yaml

@@ -3,6 +3,7 @@ apiVersion: 1
 datasources:
   - name: Prometheus
     type: prometheus
+    uid: prometheus
     access: proxy
     url: http://prometheus:9090
     isDefault: false