Handle metrics.StabilityLevel default value better.

Provide a method setDefault() to StabilityLevel type.
Update bazel by hack/update-bazel.sh

staging/src/k8s.io/component-base/metrics/BUILD

@@ -41,6 +41,7 @@ go_test(
         "counter_test.go",
         "gauge_test.go",
         "histogram_test.go",
+        "opts_test.go",
         "registry_test.go",
         "summary_test.go",
         "version_parser_test.go",

staging/src/k8s.io/component-base/metrics/opts.go

@@ -53,6 +53,16 @@ const (
 	STABLE StabilityLevel = "STABLE"
 )
 
+// setDefaults takes 'ALPHA' in case of empty.
+func (sl *StabilityLevel) setDefaults() {
+	switch *sl {
+	case "":
+		*sl = ALPHA
+	default:
+		// no-op, since we have a StabilityLevel already
+	}
+}
+
 // CounterOpts is an alias for Opts. See there for doc comments.
 type CounterOpts KubeOpts
 

staging/src/k8s.io/component-base/metrics/opts_test.go

@@ -0,0 +1,61 @@
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package metrics
+
+import (
+	"testing"
+)
+
+func TestDefaultStabilityLevel(t *testing.T) {
+	var tests = []struct {
+		name        string
+		inputValue  StabilityLevel
+		expectValue StabilityLevel
+		expectPanic bool
+	}{
+		{
+			name:        "empty should take ALPHA by default",
+			inputValue:  "",
+			expectValue: ALPHA,
+			expectPanic: false,
+		},
+		{
+			name:        "ALPHA remain unchanged",
+			inputValue:  ALPHA,
+			expectValue: ALPHA,
+			expectPanic: false,
+		},
+		{
+			name:        "STABLE remain unchanged",
+			inputValue:  STABLE,
+			expectValue: STABLE,
+			expectPanic: false,
+		},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			var stability = tc.inputValue
+
+			stability.setDefaults()
+			if stability != tc.expectValue {
+				t.Errorf("Got %s, expected: %v ", stability, tc.expectValue)
+			}
+		})
+	}
+}