scrape Longhorn metrics

apps/victoria-metrics/templates/longhorn-vmservicescrape.yaml

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+# Scrapes Longhorn's own metrics (volume robustness, node and disk capacity)
+# so the external Grafana can alert when a volume becomes degraded or faulted.
+#
+# Every longhorn-manager Pod serves /metrics on its `manager` port (9500)
+# behind the longhorn-backend Service. A VMServiceScrape scrapes each Pod
+# behind that Service, so metrics from managers on both storage workers are
+# collected, not just whichever one a load-balanced request would hit.
+#
+# This lives in the victoria-metrics wrapper chart rather than the Longhorn
+# app because it is monitoring configuration, and it depends on the
+# VMServiceScrape CRD installed by this chart's operator. Sync wave "1" makes
+# Argo CD apply it only after that CRD exists on a fresh install.
+apiVersion: operator.victoriametrics.com/v1beta1
+kind: VMServiceScrape
+metadata:
+  name: longhorn
+  namespace: monitoring
+  annotations:
+    argocd.argoproj.io/sync-wave: "1"
+spec:
+  namespaceSelector:
+    matchNames:
+      - longhorn-system
+  selector:
+    matchLabels:
+      app: longhorn-manager
+  endpoints:
+    - port: manager
+      path: /metrics
+      interval: 30s