Reduce apiserver metrics cardinality of non-core APIs

* Reduce `apiserver_request_duration_seconds_count` cardinality
by dropping series for non-core Kubernetes APIs. This is done
to match `apiserver_request_duration_seconds_count` relabeling
* These two relabels must be performed the same way to avoid
affecting new SLO calculations (upcoming)
* See kubernetes-monitoring/kubernetes-mixin#498

Related: #596

addons/prometheus/config.yaml

@@ -68,6 +68,9 @@ data:
       - source_labels: [__name__, group]
         regex: apiserver_request_duration_seconds_bucket;.+
         action: drop
+      - source_labels: [__name__, group]
+        regex: apiserver_request_duration_seconds_count;.+
+        action: drop
 
     # Scrape config for node (i.e. kubelet) /metrics (e.g. 'kubelet_'). Explore
     # metrics from a node by scraping kubelet (127.0.0.1:10250/metrics).