Allow optional VK in CR metrics

Allow optional VK in CR metrics, while only requiring the group to be
fixed. This would allow users to define custom metrics for:
* a specific API version: version is fixed, kind varies.
* all the API versions of a resource: version varies, kind is fixed.
* all the APIs part of an API group: version varies, kind varies.

Signed-off-by: Pranshu Srivastava <rexagod@gmail.com>

pkg/customresourcestate/config.go

@@ -21,6 +21,9 @@ import (
 	"strings"
 
 	"github.com/gobuffalo/flect"
+	v1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/client-go/discovery"
+	"k8s.io/client-go/rest"
 	"k8s.io/klog/v2"
 
 	"k8s.io/kube-state-metrics/v2/pkg/customresource"
@@ -161,6 +164,37 @@ type ConfigDecoder interface {
 	Decode(v interface{}) (err error)
 }
 
+// generateResources generates the set of possible resources from the varying V and, or K of the original config.
+func generateResources(resources []Resource, config *rest.Config) {
+	d := discovery.NewDiscoveryClientForConfigOrDie(config)
+	var found []v1.APIResource
+	var got *v1.APIResourceList
+	var err error
+	for _, resource := range resources {
+		gvk := resource.GroupVersionKind
+		switch {
+		// - groupVersionKind:
+		//	 group: "apps"
+		case gvk.Version == "" && gvk.Kind == "":
+		// - groupVersionKind:
+		//	 group: "apps"
+		//	 kind: "Deployment"
+		case gvk.Version == "" && gvk.Kind != "":
+		// - groupVersionKind:
+		//	 group: "apps"
+		//	 version: "v1"
+		case gvk.Version != "" && gvk.Kind == "":
+			got, err = d.ServerResourcesForGroupVersion(gvk.Group + "/" + gvk.Version)
+			if err != nil {
+				klog.Errorf("Error getting resources for groupVersion %s/%s: %v", gvk.Group, gvk.Version, err)
+				return
+			}
+		}
+		found = append(found, got.APIResources...)
+	}
+	// Convert each found element into a Resource.
+}
+
 // FromConfig decodes a configuration source into a slice of customresource.RegistryFactory that are ready to use.
 func FromConfig(decoder ConfigDecoder) ([]customresource.RegistryFactory, error) {
 	var crconfig Metrics