Implement stable metric validation and verification

Based on KEP:
https://github.com/kubernetes/enhancements/blob/master/keps/sig-instrumentation/20190605-metrics-validation-and-verification.md

Add //test/instrumentation:stable_metric_test that compares metrics
in source code to those available in
"test/instrumentation/testdata/stable-metrics-list.yaml".

test/instrumentation/metric.go

@@ -0,0 +1,53 @@
+/*
+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 main
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	counterMetricType   = "Counter"
+	gaugeMetricType     = "Gauge"
+	histogramMetricType = "Histogram"
+)
+
+type metric struct {
+	Name              string    `yaml:"name"`
+	Subsystem         string    `yaml:"subsystem,omitempty"`
+	Namespace         string    `yaml:"namespace,omitempty"`
+	Help              string    `yaml:"help,omitempty"`
+	Type              string    `yaml:"type,omitempty"`
+	DeprecatedVersion string    `yaml:"deprecatedVersion,omitempty"`
+	StabilityLevel    string    `yaml:"stabilityLevel,omitempty"`
+	Labels            []string  `yaml:"labels,omitempty"`
+	Buckets           []float64 `yaml:"buckets,omitempty"`
+}
+
+func (m metric) buildFQName() string {
+	return prometheus.BuildFQName(m.Namespace, m.Subsystem, m.Name)
+}
+
+type byFQName []metric
+
+func (ms byFQName) Len() int { return len(ms) }
+func (ms byFQName) Less(i, j int) bool {
+	return ms[i].buildFQName() < ms[j].buildFQName()
+}
+func (ms byFQName) Swap(i, j int) {
+	ms[i], ms[j] = ms[j], ms[i]
+}