Add a GCE health checks example

examples/health-checks/gce.md

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-# Health checks for the GCE controller
-
-Placeholder

examples/health-checks/gce/README.md

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+# Simple HTTP health check example
+
+The GCE Ingress controller adopts the readiness probe from the matching endpoints, provided the readiness probe doesn't require HTTPS or special headers.
+
+Create the following app:
+```console
+$ kubectl create -f health_check_app.yaml
+replicationcontroller "echoheaders" created
+You have exposed your service on an external port on all nodes in your
+cluster.  If you want to expose this service to the external internet, you may
+need to set up firewall rules for the service port(s) (tcp:31165) to serve traffic.
+
+See http://releases.k8s.io/HEAD/docs/user-guide/services-firewalls.md for more details.
+service "echoheadersx" created
+You have exposed your service on an external port on all nodes in your
+cluster.  If you want to expose this service to the external internet, you may
+need to set up firewall rules for the service port(s) (tcp:31020) to serve traffic.
+
+See http://releases.k8s.io/HEAD/docs/user-guide/services-firewalls.md for more details.
+service "echoheadersy" created
+ingress "echomap" created
+```
+
+You should soon find an Ingress that is backed by a GCE Loadbalancer.
+
+```console
+$ kubectl describe ing echomap
+Name:			echomap
+Namespace:		default
+Address:		107.178.255.228
+Default backend:	default-http-backend:80 (10.180.0.9:8080,10.240.0.2:8080)
+Rules:
+  Host		Path	Backends
+  ----		----	--------
+  foo.bar.com
+    		/foo 	echoheadersx:80 (<none>)
+  bar.baz.com
+    		/bar 	echoheadersy:80 (<none>)
+    		/foo 	echoheadersx:80 (<none>)
+Annotations:
+  target-proxy:		k8s-tp-default-echomap--a9d60e8176d933ee
+  url-map:		k8s-um-default-echomap--a9d60e8176d933ee
+  backends:		{"k8s-be-31020--a9d60e8176d933ee":"HEALTHY","k8s-be-31165--a9d60e8176d933ee":"HEALTHY","k8s-be-31686--a9d60e8176d933ee":"HEALTHY"}
+  forwarding-rule:	k8s-fw-default-echomap--a9d60e8176d933ee
+Events:
+  FirstSeen	LastSeen	Count	From				SubobjectPath	Type		Reason	Message
+  ---------	--------	-----	----				-------------	--------	------	-------
+  17m		17m		1	{loadbalancer-controller }			Normal		ADD	default/echomap
+  15m		15m		1	{loadbalancer-controller }			Normal		CREATE	ip: 107.178.255.228
+
+$ curl 107.178.255.228/foo -H 'Host:foo.bar.com'
+CLIENT VALUES:
+client_address=10.240.0.5
+command=GET
+real path=/foo
+query=nil
+request_version=1.1
+request_uri=http://foo.bar.com:8080/foo
+...
+```
+
+You can confirm the health check endpoint point it's using one of 2 ways:
+* Through the cloud console: compute > health checks > lookup your health check. It takes the form k8s-be-nodePort-hash, where nodePort in the example above is 31165 and 31020, as shown by the kubectl output.
+* Through gcloud: Run `gcloud compute http-health-checks list`
+
+## Limitations
+
+A few points to note:
+* The readiness probe must be exposed on the port matching the `servicePort` specified in the Ingress
+* The readiness probe cannot have special requirements, like headers or HTTPS
+* The probe timeouts are translated to GCE health check timeouts
+* You must create the pods backing the endpoints with the given readiness probe. This *will not* work if you update the replication controller with a different readiness probe.
+
+

examples/health-checks/gce/health_check_app.yaml

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+apiVersion: v1
+kind: ReplicationController
+metadata:
+  name: echoheaders
+spec:
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        app: echoheaders
+    spec:
+      containers:
+      - name: echoheaders
+        image: gcr.io/google_containers/echoserver:1.4
+        ports:
+        - containerPort: 8080
+        readinessProbe:
+          httpGet:
+            path: /healthz
+            port: 8080
+          periodSeconds: 1
+          timeoutSeconds: 1
+          successThreshold: 1
+          failureThreshold: 10
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: echoheadersx
+  labels:
+    app: echoheaders
+spec:
+  type: NodePort
+  ports:
+  - port: 80
+    targetPort: 8080
+    protocol: TCP
+    name: http
+  selector:
+    app: echoheaders
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: echoheadersy
+  labels:
+    app: echoheaders
+spec:
+  type: NodePort
+  ports:
+  - port: 80
+    targetPort: 8080
+    protocol: TCP
+    name: http
+  selector:
+    app: echoheaders
+---
+apiVersion: extensions/v1beta1
+kind: Ingress
+metadata:
+  name: echomap
+spec:
+  rules:
+  - host: foo.bar.com
+    http:
+      paths:
+      - path: /foo
+        backend:
+          serviceName: echoheadersx
+          servicePort: 80
+  - host: bar.baz.com
+    http:
+      paths:
+      - path: /bar
+        backend:
+          serviceName: echoheadersy
+          servicePort: 80
+      - path: /foo
+        backend:
+          serviceName: echoheadersx
+          servicePort: 80
+