Improve apiserver storage size metric to allow it's graduation

Change name to make it compliant with prometheus guidelines.
Calculate it on demand instead of periodic to comply with prometheus standards.
Replace "endpoint" with "server" label to make it semantically consistent with storage factory

test/integration/metrics/metrics_test.go

@@ -76,6 +76,28 @@ func TestAPIServerProcessMetrics(t *testing.T) {
 	})
 }
 
+func TestAPIServerStorageMetrics(t *testing.T) {
+	s := kubeapiservertesting.StartTestServerOrDie(t, nil, nil, framework.SharedEtcd())
+	defer s.TearDownFn()
+
+	metrics, err := scrapeMetrics(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	samples, ok := metrics["apiserver_storage_size_bytes"]
+	if !ok {
+		t.Fatalf("apiserver_storage_size_bytes metric not exposed")
+	}
+	if len(samples) != 1 {
+		t.Fatalf("Unexpected number of samples in apiserver_storage_size_bytes")
+	}
+
+	if samples[0].Value == -1 {
+		t.Errorf("Unexpected non-zero apiserver_storage_size_bytes, got: %s", samples[0].Value)
+	}
+}
+
 func TestAPIServerMetrics(t *testing.T) {
 	s := kubeapiservertesting.StartTestServerOrDie(t, nil, nil, framework.SharedEtcd())
 	defer s.TearDownFn()