Introduce testutil package to support metrics testing.

staging/src/k8s.io/component-base/metrics/legacyregistry/registry.go

@@ -26,7 +26,7 @@ import (
 var (
 	defaultRegistry = metrics.NewKubeRegistry()
 	// DefaultGatherer exposes the global registry gatherer
-	DefaultGatherer prometheus.Gatherer = defaultRegistry
+	DefaultGatherer metrics.Gatherer = defaultRegistry
 )
 
 func init() {

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

@@ -0,0 +1,33 @@
+/*
+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 testutil
+
+import (
+	"io"
+
+	"github.com/prometheus/client_golang/prometheus/testutil"
+
+	"k8s.io/component-base/metrics"
+)
+
+// GatherAndCompare gathers all metrics from the provided Gatherer and compares
+// it to an expected output read from the provided Reader in the Prometheus text
+// exposition format. If any metricNames are provided, only metrics with those
+// names are compared.
+func GatherAndCompare(g metrics.Gatherer, expected io.Reader, metricNames ...string) error {
+	return testutil.GatherAndCompare(g, expected, metricNames...)
+}

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

@@ -79,3 +79,9 @@ type PromRegistry interface {
 	Unregister(prometheus.Collector) bool
 	Gather() ([]*dto.MetricFamily, error)
 }
+
+// Gatherer is the interface for the part of a registry in charge of gathering
+// the collected metrics into a number of MetricFamilies.
+type Gatherer interface {
+	prometheus.Gatherer
+}