Use recommended instance label for Prometheus/Alertmanager resources

manifests/alertmanager-alertmanager.yaml

@@ -2,8 +2,8 @@ apiVersion: monitoring.coreos.com/v1
 kind: Alertmanager
 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
@@ -15,8 +15,8 @@ spec:
     kubernetes.io/os: linux
   podMetadata:
     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