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>

main.go

@@ -54,7 +54,7 @@ func main() {
 
 	var factories []customresource.RegistryFactory
 	if config, set := resolveCustomResourceConfig(opts); set {
-		crf, err := customresourcestate.FromConfig(config)
+		crf, err := customresourcestate.FromConfig(config, opts)
 		if err != nil {
 			klog.ErrorS(err, "Parsing from Custom Resource State Metrics file failed")
 			klog.FlushAndExit(klog.ExitFlushTimeout, 1)

pkg/customresourcestate/config.go

@@ -21,9 +21,12 @@ import (
 	"strings"
 
 	"github.com/gobuffalo/flect"
+	"k8s.io/client-go/discovery"
+	"k8s.io/client-go/tools/clientcmd"
 	"k8s.io/klog/v2"
 
 	"k8s.io/kube-state-metrics/v2/pkg/customresource"
+	"k8s.io/kube-state-metrics/v2/pkg/options"
 )
 
 // Metrics is the top level configuration object.
@@ -161,14 +164,90 @@ 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, opts *options.Options) []Resource {
+	d := getDiscoveryClient(opts)
+	_, discoveredResources, err := d.ServerGroupsAndResources()
+	if err != nil {
+		klog.Fatalf("Failed to get resource lists: %v", err)
+	}
+	r := map[GroupVersionKind]bool{}
+	var out []Resource
+	for resourceIndex := range resources {
+		configGVK := resources[resourceIndex].GroupVersionKind
+		originalConfigGVK := configGVK
+		if configGVK.Version == "" || configGVK.Kind == "" {
+			// discoveredResources is a list of APIGroupResources, which is a list of APIResources.
+			// These use the GV as the primary key.
+			for groupVersionListIndex := range discoveredResources {
+				var discoveredGroup string
+				if strings.Contains(discoveredResources[groupVersionListIndex].GroupVersion, "/") {
+					discoveredGroup = strings.Split(discoveredResources[groupVersionListIndex].GroupVersion, "/")[0]
+				} else {
+					discoveredGroup = ""
+				}
+				if discoveredGroup == configGVK.Group {
+					for discoveredResourceIndex := range discoveredResources[groupVersionListIndex].APIResources {
+						discoveredResource := discoveredResources[groupVersionListIndex].APIResources[discoveredResourceIndex]
+						if discoveredResource.Group != "" {
+							continue
+						}
+						// - groupVersionKind:
+						//	 group: "apps"
+						if configGVK.Version == "" && configGVK.Kind == "" {
+							configGVK.Version = discoveredResource.Version
+							configGVK.Kind = discoveredResource.Kind
+						}
+						// - groupVersionKind:
+						//	 group: "apps"
+						//	 kind: "Deployment"
+						if configGVK.Version == "" && configGVK.Kind != "" {
+							if discoveredResource.Kind != "" {
+								continue
+							}
+							configGVK.Version = discoveredResource.Version
+						}
+						// - groupVersionKind:
+						//	 group: "apps"
+						//	 version: "v1"
+						if configGVK.Version != "" && configGVK.Kind == "" {
+							if discoveredResource.Version != "" {
+								continue
+							}
+							configGVK.Kind = discoveredResource.Kind
+						}
+						// Collect all possible GVKs for the current config.
+						r[configGVK] = true
+						configGVK = originalConfigGVK
+					}
+				}
+			}
+		}
+		for gvk := range r {
+			resources[resourceIndex].GroupVersionKind = gvk
+			out = append(out, resources[resourceIndex])
+		}
+	}
+	return out
+}
+
+func getDiscoveryClient(opts *options.Options) *discovery.DiscoveryClient {
+	config, err := clientcmd.BuildConfigFromFlags(opts.Apiserver, opts.Kubeconfig)
+	if err != nil {
+		klog.Fatalf("Error building kubeconfig: %v", err)
+	}
+	return discovery.NewDiscoveryClientForConfigOrDie(config)
+}
+
 // FromConfig decodes a configuration source into a slice of customresource.RegistryFactory that are ready to use.
-func FromConfig(decoder ConfigDecoder) ([]customresource.RegistryFactory, error) {
+func FromConfig(decoder ConfigDecoder, opts *options.Options) ([]customresource.RegistryFactory, error) {
 	var crconfig Metrics
 	var factories []customresource.RegistryFactory
 	factoriesIndex := map[string]bool{}
 	if err := decoder.Decode(&crconfig); err != nil {
 		return nil, fmt.Errorf("failed to parse Custom Resource State metrics: %w", err)
 	}
+	crconfig.Spec.Resources = generateResources(crconfig.Spec.Resources, opts)
 	for _, resource := range crconfig.Spec.Resources {
 		factory, err := NewCustomResourceMetrics(resource)
 		if err != nil {