pkg/api: fix singular name

With the 1.27 Kubernetes upgrade, a new GetSingularName method needed to
be implemented by the metrics rest.Storage. While doing it, we set a
value for the singular name of the resources when it used to be empty.
Due to that, the kube-apiserver stop guessing the resource name based on
its Kind, which broken kubectl get PodMetrics/NodeMetrics.

To fix that, we need to blank out the singular name of the resources,
otherwise we will be stuck with the following commands:
- kubectl get pod.metric
- kubectl get node.metric
Instead of the more commonly used:
- kubectl get PodMetrics
- kubectl get NodeMetrics

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

pkg/api/node.go

@@ -178,5 +178,5 @@ func (m *nodeMetrics) NamespaceScoped() bool {
 
 // GetSingularName implements rest.SingularNameProvider interface
 func (m *nodeMetrics) GetSingularName() string {
-	return "node"
+	return ""
 }

pkg/api/pod.go

@@ -185,5 +185,5 @@ func (m *podMetrics) NamespaceScoped() bool {
 
 // GetSingularName implements rest.SingularNameProvider interface
 func (m *podMetrics) GetSingularName() string {
-	return "pod"
+	return ""
 }