add back the link to stable metrics list

Author: Han Kang

content/en/docs/concepts/cluster-administration/system-metrics.md

@@ -174,4 +174,5 @@ Here is an example:
 ## {{% heading "whatsnext" %}}
 
 * Read about the [Prometheus text format](https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md#text-based-format) for metrics
+* See the list of [stable Kubernetes metrics](https://github.com/kubernetes/kubernetes/blob/master/test/instrumentation/testdata/stable-metrics-list.yaml)
 * Read about the [Kubernetes deprecation policy](/docs/reference/using-api/deprecation-policy/#deprecating-a-feature-or-behavior)