Add documentation for metrics.

Author: neunhoef

docs/Manual/Deployment/Kubernetes/DeploymentResource.md

@@ -354,6 +354,50 @@ There are two magic values for the secret name:
 - `None` specifies no action. This disables root password randomization. This is the default value. (Thus the root password is empty - not recommended)
 - `Auto` specifies automatic name generation, which is `<deploymentname>-root-password`. 
 
+### `spec.metrics.enabled: bool`
+
+If this is set to `true`, the operator runs a sidecar container for
+every DBserver pod and every coordinator pod. The sidecar container runs
+the ArangoDB-exporter and exposes metrics of the corresponding `arangod`
+instance in Prometheus format on port 9101 under path `/metrics`. You
+also have to specify a string for `spec.metrics.image`, which is the
+Docker image name of the `arangodb-exporter`. At the time of this
+writing you should use `arangodb/arangodb-exporter:0.1.6`. See [this
+repository](https://github.com/arangodb-helper/arangodb-exporter) for
+the latest version.
+
+In addition to the sidecar containers the operator will deploy a service
+to access the exporter ports (from within the k8s cluster), and a
+resource of type `ServiceMonitor`, provided the corresponding custom
+resource definition is deployed in the k8s cluster. If you are running
+Prometheus in the same k8s cluster with the Prometheus operator, this
+will be the case. The `ServiceMonitor` will have the following labels
+set:
+
+  - `app: arangodb`
+  - `arango_deployment: YOUR_DEPLOYMENT_NAME`
+  - `context: metrics`
+  - `metrics: prometheus`
+
+This makes it possible that you configure your Prometheus deployment to
+automatically start monitoring on the available Prometheus feeds. To
+this end, you must configure the `serviceMonitorSelector` in the specs
+of your Prometheus deployment to match these labels. For example:
+
+```yaml
+  serviceMonitorSelector:
+    matchLabels:
+      metrics: prometheus
+```
+
+would automatically select all pods of all ArangoDB cluster deployments
+which have metrics enabled.
+
+### `spec.metrics.image: string`
+
+See above, this is the name of the Docker image for the ArangoDB
+exporter to expose metrics.
+
 ### `spec.<group>.count: number`
 
 This setting specifies the number of servers to start for the given group.
@@ -457,4 +501,4 @@ because servers in these groups do not need persistent storage.
 
 Please use VolumeClaimTemplate from now on. This field is not considered if 
 VolumeClaimTemplate is set. Note however, that the information in requests 
-is completely handed over to the pod in this case.
+is completely handed over to the pod in this case.

docs/Manual/Deployment/Kubernetes/Metrics.md

@@ -8,3 +8,8 @@ The metrics are exposed through HTTPS on port `8528` under path `/metrics`.
 Look at [examples/metrics](https://github.com/arangodb/kube-arangodb/tree/master/examples/metrics)
 for examples of `Services` and `ServiceMonitors` you can use to integrate
 with Prometheus through the [Prometheus-Operator by CoreOS](https://github.com/coreos/prometheus-operator).
+
+Furthermore, the operator can run sidecar containers for ArangoDB
+deployments of type Cluster which expose metrics in Prometheus format.
+Use the attribute `spec.metrics` to set this up see the [spec
+reference](./DeploymentResource.md) for details.