feat(metrics): collect Headscale, NetBird, Gitea and Immich metrics

prometheus-stack/k8s/README.md

@@ -15,3 +15,29 @@ The VictoriaMetrics Operator chart and its CRDs are installed before the
 Kubernetes manifests by the normal deploy workflow. On a cluster where the
 operator CRDs are not installed yet, CI skips the server-side dry-run of the
 `VMAgent` resource; the deploy installs the chart before applying that resource.
+
+## Application metrics
+
+The application ServiceMonitors use a 30s interval and a 10s timeout:
+
+- Headscale: the external Service points to the Compose host on port 19090.
+  A VMServiceScrape uses EndpointSlice discovery for this manually managed target.
+  The Compose configuration must bind metrics to `0.0.0.0:9090`.
+- NetBird: the combined server exports `/metrics` on port 9090. The existing
+  `server.metricsPort` setting enables the listener.
+- Gitea: `GITEA__metrics__ENABLED` enables `/metrics` on the HTTP port. The public
+  ingress excludes this path. The monitor uses the internal Service directly.
+- Immich: `IMMICH_TELEMETRY_INCLUDE=all` enables API and worker metrics on ports
+  8081 and 8082. The monitor scrapes both ports on each server replica.
+
+Deploy through the existing CI and deploy workflow. Gitea and Immich reload their
+ConfigMap changes through Reloader. Check the VMAgent targets after deployment
+and query `up{scraper="victoria",namespace=~"netbird|gitea|immich|headscale"}` in
+VictoriaMetrics. All targets should report 1.
+
+For rollback, revert the application metrics changes, run CI, and deploy the
+revert. Remove the three application ServiceMonitors and the Headscale VMServiceScrape explicitly: the deployment
+workflow applies manifests and does not prune removed resources.
+
+For Headscale rollback, remove its VMServiceScrape and Service label, restore the
+previous Compose metrics bind address, and restart only the Headscale service.