feat(shared): Add monitoring rule for database network policy

This patch allows to monitor Postgresql Instances by allowing the
monitoring namespace to access a exporter container port by default for
spilo instances.

shared/networkpolicies/allow-from-database.yaml

@@ -20,6 +20,16 @@ spec:
       protocol: TCP
     - port: 8080
       protocol: TCP
+  - from:
+    - namespaceSelector:
+        matchLabels:
+          monitoring.shivering-isles.com/network-access-required: "true"
+      podSelector:
+        matchLabels:
+          app.kubernetes.io/name: prometheus
+    ports:
+    - port: 9187
+      protocol: TCP
   podSelector:
     matchLabels:
       application: spilo