manifests: add Alertmanager manifests and discovery

The latest version of the Prometheus Operator requires Prometheus
>=v1.4.0 for the Alertmanger discovery feature.

manifests/prometheus/prometheus-k8s.yaml

@@ -5,7 +5,7 @@ metadata:
   labels:
     prometheus: k8s
 spec:
-  version: v1.3.0
+  version: v1.4.1
   resources:
     requests:
       # 2Gi is default, but won't schedule if you don't have a node with >2Gi
@@ -13,3 +13,8 @@ spec:
       # production use. This value is mainly meant for demonstration/testing
       # purposes.
       memory: 400Mi
+  alerting:
+    alertmanagers:
+    - namespace: monitoring
+      name: alertmanager-main
+      port: web