Use recommended instance label for Prometheus/Alertmanager resources

manifests/alertmanager-service.yaml

@@ -2,8 +2,8 @@ apiVersion: v1
 kind: Service
 metadata:
   labels:
-    alertmanager: main
     app.kubernetes.io/component: alert-router
+    app.kubernetes.io/instance: main
     app.kubernetes.io/name: alertmanager
     app.kubernetes.io/part-of: kube-prometheus
     app.kubernetes.io/version: 0.23.0
@@ -18,8 +18,8 @@ spec:
     port: 8080
     targetPort: reloader-web
   selector:
-    alertmanager: main
     app.kubernetes.io/component: alert-router
+    app.kubernetes.io/instance: main
     app.kubernetes.io/name: alertmanager
     app.kubernetes.io/part-of: kube-prometheus
   sessionAffinity: ClientIP