webhook: register metrics only with default stats reporter

This is a followup from [knative#2931][1]. RegisterMetrics may
register unwanted metrics if the default stats reporter is not used.

[1]: knative#2931

injection/sharedmain/main.go

@@ -289,14 +289,6 @@ func MainWithConfig(ctx context.Context, component string, cfg *rest.Config, cto
 	// and pass them in.
 	var wh *webhook.Webhook
 	if len(webhooks) > 0 {
-		// Register webhook metrics
-		opts := webhook.GetOptions(ctx)
-		if opts != nil {
-			webhook.RegisterMetrics(opts.StatsReporterOptions...)
-		} else {
-			webhook.RegisterMetrics()
-		}
-
 		wh, err = webhook.New(ctx, webhooks)
 		if err != nil {
 			logger.Fatalw("Failed to create webhook", zap.Error(err))

webhook/webhook.go

@@ -25,6 +25,7 @@ import (
 	"log"
 	"net"
 	"net/http"
+	"sync"
 	"time"
 
 	// Injection stuff
@@ -40,6 +41,10 @@ import (
 	"knative.dev/pkg/system"
 )
 
+var (
+	metricsOnce sync.Once
+)
+
 // Options contains the configuration for the webhook
 type Options struct {
 	// TLSMinVersion contains the minimum TLS version that is acceptable to communicate with the API server.
@@ -147,6 +152,8 @@ func New(
 	logger := logging.FromContext(ctx)
 
 	if opts.StatsReporter == nil {
+		// Register webhook metrics
+		metricsOnce.Do(func() { RegisterMetrics(opts.StatsReporterOptions...) })
 		reporter, err := NewStatsReporter(opts.StatsReporterOptions...)
 		if err != nil {
 			return nil, err