add aggregated /apis handler for genericapiserver

so that aggregated-apiservers can also take advantage. discovered by e2e tests with feature enabled

staging/src/k8s.io/apiserver/pkg/server/config.go

@@ -35,6 +35,7 @@ import (
 	jsonpatch "github.com/evanphx/json-patch"
 	"github.com/google/uuid"
 
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/runtime/serializer"
@@ -931,7 +932,12 @@ func installAPI(s *GenericAPIServer, c *Config) {
 	routes.Version{Version: c.Version}.Install(s.Handler.GoRestfulContainer)
 
 	if c.EnableDiscovery {
-		s.Handler.GoRestfulContainer.Add(s.DiscoveryGroupManager.WebService())
+		if utilfeature.DefaultFeatureGate.Enabled(genericfeatures.AggregatedDiscoveryEndpoint) {
+			wrapped := discoveryendpoint.WrapAggregatedDiscoveryToHandler(s.DiscoveryGroupManager, s.AggregatedDiscoveryGroupManager)
+			s.Handler.GoRestfulContainer.Add(wrapped.GenerateWebService("/apis", metav1.APIGroupList{}))
+		} else {
+			s.Handler.GoRestfulContainer.Add(s.DiscoveryGroupManager.WebService())
+		}
 	}
 	if c.FlowControl != nil && utilfeature.DefaultFeatureGate.Enabled(genericfeatures.APIPriorityAndFairness) {
 		c.FlowControl.Install(s.Handler.NonGoRestfulMux)