Added documentation for Metrics Server (#5560)

Author: piosz

_data/tasks.yml

@@ -80,6 +80,7 @@ toc:
 
 - title: Monitor, Log, and Debug
   section:
+  - docs/tasks/debug-application-cluster/core-metrics-pipeline.md
   - docs/tasks/debug-application-cluster/resource-usage-monitoring.md
   - docs/tasks/debug-application-cluster/get-shell-running-container.md
   - docs/tasks/debug-application-cluster/monitor-node-health.md

docs/tasks/debug-application-cluster/core-metrics-pipeline.md

@@ -0,0 +1,44 @@
+---
+approvers:
+- fgrzadkowski
+- piosz
+title: Core metrics pipeline
+---
+
+Starting from Kubernetes 1.8, resource usage metrics, such as container CPU and memory usage,
+are available in Kubernetes through the Metrics API. These metrics can be either accessed directly
+by user, for example by using `kubectl top` command, or used by a controller in the cluster, e.g.
+Horizontal Pod Autoscaler, to make decisions.
+
+## The Metrics API
+
+Through the Metrics API you can get the amount of resource currently used
+by a given node or a given pod. This API doesn't store the metric values,
+so it's not possible for example to get the amount of resources used by a
+given node 10 minutes ago.
+
+The API no different from any other API:
+
+- it is discoverable through the same endpoint as the other Kubernetes APIs under `/apis/metrics.k8s.io/` path
+- it offers the same security, scalability and reliability guarantees
+
+The API is defined in [k8s.io/metrics](https://github.com/kubernetes/metrics/blob/master/pkg/apis/metrics/v1beta1/types.go)
+repository. You can find more information about the API there.
+
+**Note:** The API requires metrics server to be deployed in the cluster. Otherwise it will be not available.
+
+## Metrics Server
+
+[Metrics Server](https://github.com/kubernetes-incubator/metrics-server) is a cluster-wide aggregator of resource usage data.
+Starting from Kubernetes 1.8 it's deployed by default in clusters created by `kube-up.sh` script
+as a Deployment object. If you use a different Kubernetes setup mechanism you can deploy it using the provided
+[deployment yamls](https://github.com/kubernetes-incubator/metrics-server/tree/master/deploy).
+It's supported in Kubernetes 1.7+ (see details below).
+
+Metric server collects metrics from the Summary API, exposed by [Kubelet](/docs/admin/kubelet/) on each node.
+
+Metrics Server registered in the main API server through
+[Kubernetes aggregator](https://kubernetes.io/docs/concepts/api-extension/apiserver-aggregation/),
+which was introduced in Kubernetes 1.7.
+
+Learn more about the metrics server in [the design doc](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/instrumentation/metrics-server.md).