Updates for grafana stack

roles/observability/templates/prometheus.yml.j2

@@ -0,0 +1,13 @@
+# Managed by Ansible (roles/observability). Do not edit by hand.
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+
+# Alloy on each host pushes metrics via remote_write.
+# Local scrape keeps Prometheus self-health visible.
+scrape_configs:
+  - job_name: prometheus
+    static_configs:
+      - targets: ["localhost:9090"]
+        labels:
+          host: "{{ inventory_hostname }}"