use noopRegistry for default global legacy prom registry and expose an http handler

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

@@ -11,6 +11,8 @@ go_library(
         "//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",
+        "//vendor/github.com/prometheus/client_golang/prometheus/promhttp:go_default_library",
+        "//vendor/github.com/prometheus/client_model/go:go_default_library",
     ],
 )
 

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

@@ -18,17 +18,31 @@ package legacyregistry
 
 import (
 	"fmt"
+	"net/http"
 	"sync"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	dto "github.com/prometheus/client_model/go"
+
 	apimachineryversion "k8s.io/apimachinery/pkg/version"
 	"k8s.io/component-base/metrics"
 )
 
-var globalRegistryFactory = metricsRegistryFactory{
+var (
+	globalRegistryFactory = metricsRegistryFactory{
 		registerQueue:     make([]metrics.KubeCollector, 0),
 		mustRegisterQueue: make([]metrics.KubeCollector, 0),
-}
+		globalRegistry:    noopRegistry{},
+	}
+)
+
+type noopRegistry struct{}
+
+func (noopRegistry) Register(metrics.KubeCollector) error  { return nil }
+func (noopRegistry) MustRegister(...metrics.KubeCollector) {}
+func (noopRegistry) Unregister(metrics.KubeCollector) bool { return true }
+func (noopRegistry) Gather() ([]*dto.MetricFamily, error)  { return nil, nil }
 
 type metricsRegistryFactory struct {
 	globalRegistry    metrics.KubeRegistry
@@ -38,6 +52,13 @@ type metricsRegistryFactory struct {
 	mustRegisterQueue []metrics.KubeCollector
 }
 
+// HandlerForGlobalRegistry returns a http handler for the global registry. This
+// allows us to return a handler for the global registry without having to expose
+// the global registry itself directly.
+func HandlerForGlobalRegistry(opts promhttp.HandlerOpts) http.Handler {
+	return promhttp.HandlerFor(globalRegistryFactory.globalRegistry, opts)
+}
+
 // SetRegistryFactoryVersion sets the kubernetes version information for all
 // subsequent metrics registry initializations. Only the first call has an effect.
 // If a version is not set, then metrics registry creation will no-opt
@@ -77,11 +98,12 @@ func Register(c metrics.KubeCollector) error {
 	globalRegistryFactory.registrationLock.Lock()
 	defer globalRegistryFactory.registrationLock.Unlock()
 
-	if globalRegistryFactory.kubeVersion != nil {
+	if globalRegistryFactory.globalRegistry == (noopRegistry{}) {
-		return globalRegistryFactory.globalRegistry.Register(c)
-	}
 		globalRegistryFactory.registerQueue = append(globalRegistryFactory.registerQueue, c)
 		return nil
+	}
+
+	return globalRegistryFactory.globalRegistry.Register(c)
 }
 
 // MustRegister works like Register but registers any number of
@@ -91,11 +113,12 @@ func MustRegister(cs ...metrics.KubeCollector) {
 	globalRegistryFactory.registrationLock.Lock()
 	defer globalRegistryFactory.registrationLock.Unlock()
 
-	if globalRegistryFactory.kubeVersion != nil {
+	if globalRegistryFactory.globalRegistry == (noopRegistry{}) {
-		globalRegistryFactory.globalRegistry.MustRegister(cs...)
-		return
-	}
 		for _, c := range cs {
 			globalRegistryFactory.mustRegisterQueue = append(globalRegistryFactory.mustRegisterQueue, c)
 		}
+		return
+	}
+	globalRegistryFactory.globalRegistry.MustRegister(cs...)
+	return
 }

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

@@ -150,6 +150,7 @@ func TestDeferredRegister(t *testing.T) {
 	globalRegistryFactory = metricsRegistryFactory{
 		registerQueue:     make([]metrics.KubeCollector, 0),
 		mustRegisterQueue: make([]metrics.KubeCollector, 0),
+		globalRegistry:    noopRegistry{},
 	}
 	var err error
 	err = Register(alphaDeprecatedCounter)
@@ -179,6 +180,7 @@ func TestDeferredMustRegister(t *testing.T) {
 	globalRegistryFactory = metricsRegistryFactory{
 		registerQueue:     make([]metrics.KubeCollector, 0),
 		mustRegisterQueue: make([]metrics.KubeCollector, 0),
+		globalRegistry:    noopRegistry{},
 	}
 	MustRegister(alphaDeprecatedCounter)