Merge pull request kubernetes#70289 from CharlyF/listing-datadog-clus…

…ter-agent

Adding Datadog Cluster Agent to the list of Custom Metrics Providers

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

@@ -24,3 +24,7 @@ They are listed here for convenience.***
 
 - [Google Stackdriver (coming
   soon)](https://github.com/GoogleCloudPlatform/k8s-stackdriver)
+
+- [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.