Add kube-metrics-adapter to metrics implementations

staging/src/k8s.io/metrics/IMPLEMENTATIONS.md

@@ -27,4 +27,6 @@ They are listed here for convenience.***
 
 - [Datadog Cluster Agent](https://github.com/DataDog/datadog-agent/blob/c4f38af1897bac294d8fed6285098b14aafa6178/docs/cluster-agent/CUSTOM_METRICS_SERVER.md).
   Implementation of the external metrics provider, using Datadog as a backend for the metrics.
-  Coming soon: Implementation of the custom metrics provider to support in-cluster metrics collected by the Datadog Agents.
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+  Coming soon: Implementation of the custom metrics provider to support in-cluster metrics collected by the Datadog Agents.
+
+- [Kube Metrics Adapter](https://github.com/zalando-incubator/kube-metrics-adapter). A general purpose metrics adapter for Kubernetes that can collect and serve custom and external metrics for Horizontal Pod Autoscaling.
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