update status on metric stability framework

Author: Han Kang

keps/sig-instrumentation/1209-metrics-stability/kep.yaml

@@ -13,7 +13,7 @@ participating-sigs:
   - sig-cluster-lifecycle
   - sig-cloud-provider
   - sig-testing
-status: implementable
+status: implemented
 creation-date: 2019-04-04
 reviewers:
   - "@brancz" # instrumentation