prometheus: Add ability to whitelist Kubernetes labels

manifests/examples/example-app/example-app.yaml

 kind: Service
 apiVersion: v1
-metadata: 
+metadata:
   name: example-app
   labels:
     tier: frontend
   namespace: default
-spec: 
-  selector: 
-    app: example-app 
+spec:
+  selector:
+    app: example-app
   ports:
   - name: web
     protocol: TCP

manifests/examples/example-app/servicemonitor-frontend.yaml

@@ -9,6 +9,8 @@ spec:
   selector:
     matchLabels:
       tier: frontend
+  targetLabels:
+    - tier
   endpoints:
   - port: web
     interval: 10s