port over apiserver metric handler to use the instrumentation library

Change-Id: Ifabb726120c1c4f90ff3e4d71fdbba468fe74e8a

Author: Han Kang

staging/src/k8s.io/apiserver/pkg/server/routes/metrics.go

@@ -17,9 +17,6 @@ limitations under the License.
 package routes
 
 import (
-	"io"
-	"net/http"
-
 	apimetrics "k8s.io/apiserver/pkg/endpoints/metrics"
 	"k8s.io/apiserver/pkg/server/mux"
 	etcd3metrics "k8s.io/apiserver/pkg/storage/etcd3/metrics"
@@ -43,18 +40,7 @@ type MetricsWithReset struct{}
 // Install adds the MetricsWithReset handler
 func (m MetricsWithReset) Install(c *mux.PathRecorderMux) {
 	register()
-	defaultMetricsHandler := legacyregistry.Handler().ServeHTTP
-	c.HandleFunc("/metrics", func(w http.ResponseWriter, req *http.Request) {
-		if req.Method == "DELETE" {
-			apimetrics.Reset()
-			etcd3metrics.Reset()
-			flowcontrolmetrics.Reset()
-
-			io.WriteString(w, "metrics reset\n")
-			return
-		}
-		defaultMetricsHandler(w, req)
-	})
+	c.Handle("/metrics", legacyregistry.HandlerWithReset())
 }
 
 // register apiserver and etcd metrics