preload metrics for legacyregistry for backwards compatibility

The global prometheus registry comes preloaded with process and go
metrics. Since these are not under kubernetes control, they can't be
considered stable. However, we can make a best effort to maintain
backwards compatibility by preloading the same metrics.

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

@@ -10,6 +10,7 @@ go_library(
     deps = [
         "//staging/src/k8s.io/apimachinery/pkg/version:go_default_library",
         "//staging/src/k8s.io/component-base/metrics:go_default_library",
+        "//vendor/github.com/prometheus/client_golang/prometheus:go_default_library",
     ],
 )
 

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

@@ -18,9 +18,11 @@ package legacyregistry
 
 import (
 	"fmt"
+	"sync"
+
+	"github.com/prometheus/client_golang/prometheus"
 	apimachineryversion "k8s.io/apimachinery/pkg/version"
 	"k8s.io/component-base/metrics"
-	"sync"
 )
 
 var globalRegistryFactory = metricsRegistryFactory{
@@ -51,7 +53,11 @@ func SetRegistryFactoryVersion(ver apimachineryversion.Info) []error {
 		return nil
 	}
 	registrationErrs := make([]error, 0)
-	globalRegistryFactory.globalRegistry = metrics.NewKubeRegistry(ver)
+	preloadedMetrics := []prometheus.Collector{
+		prometheus.NewGoCollector(),
+		prometheus.NewProcessCollector(prometheus.ProcessCollectorOpts{}),
+	}
+	globalRegistryFactory.globalRegistry = metrics.NewPreloadedKubeRegistry(ver, preloadedMetrics...)
 	globalRegistryFactory.kubeVersion = &ver
 	for _, c := range globalRegistryFactory.registerQueue {
 		err := globalRegistryFactory.globalRegistry.Register(c)

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

@@ -193,3 +193,33 @@ func TestDeferredMustRegister(t *testing.T) {
 		},
 		"Did not panic even though we expected it.")
 }
+
+func TestPreloadedMetrics(t *testing.T) {
+	// reset the global registry for this test.
+	globalRegistryFactory = metricsRegistryFactory{
+		registerQueue:     make([]metrics.KubeCollector, 0),
+		mustRegisterQueue: make([]metrics.KubeCollector, 0),
+	}
+
+	SetRegistryFactoryVersion(apimachineryversion.Info{
+		Major:      "1",
+		Minor:      "15",
+		GitVersion: "v1.15.0-alpha-1.12345",
+	})
+	// partial list of some preregistered metrics we expect
+	expectedMetricNames := []string{"go_gc_duration_seconds", "process_start_time_seconds"}
+
+	mf, err := globalRegistryFactory.globalRegistry.Gather()
+	if err != nil {
+		t.Errorf("Got unexpected error %v ", err)
+	}
+	metricNames := map[string]struct{}{}
+	for _, f := range mf {
+		metricNames[f.GetName()] = struct{}{}
+	}
+	for _, expectedMetric := range expectedMetricNames {
+		if _, ok := metricNames[expectedMetric]; !ok {
+			t.Errorf("Expected %v to be preregistered", expectedMetric)
+		}
+	}
+}

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

@@ -94,3 +94,18 @@ func NewKubeRegistry(v apimachineryversion.Info) KubeRegistry {
 		version:      parseVersion(v),
 	}
 }
+
+// NewPreloadedKubeRegistry creates a new Registry with preloaded prometheus collectors
+// already registered. This is exposed specifically to maintain backwards
+// compatibility with the global prometheus registry.
+//
+// Deprecated
+func NewPreloadedKubeRegistry(v apimachineryversion.Info, cs ...prometheus.Collector) KubeRegistry {
+	registry := &kubeRegistry{
+		PromRegistry: prometheus.NewRegistry(),
+		version:      parseVersion(v),
+	}
+
+	registry.PromRegistry.MustRegister(cs...)
+	return registry
+}