Upgrade boilerplate to latest

The latest boilerplate comes with a lot of simplifications and helpers
that let us reduce the amount of code written.

vendor/github.com/kubernetes-incubator/custom-metrics-apiserver/pkg/provider/helpers/helpers.go

@@ -0,0 +1,106 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package helpers
+
+import (
+	"fmt"
+
+	apimeta "k8s.io/apimachinery/pkg/api/meta"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
+	"k8s.io/apimachinery/pkg/labels"
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/apimachinery/pkg/types"
+	"k8s.io/client-go/dynamic"
+	"k8s.io/metrics/pkg/apis/custom_metrics"
+
+	"github.com/kubernetes-incubator/custom-metrics-apiserver/pkg/provider"
+)
+
+// ResourceFor attempts to resolve a single qualified resource for the given metric.
+// You can use this to resolve a particular piece of CustomMetricInfo to the underlying
+// resource that it describes, so that you can list matching objects in the cluster.
+func ResourceFor(mapper apimeta.RESTMapper, info provider.CustomMetricInfo) (schema.GroupVersionResource, error) {
+	fullResources, err := mapper.ResourcesFor(info.GroupResource.WithVersion(""))
+	if err == nil && len(fullResources) == 0 {
+		err = fmt.Errorf("no fully versioned resources known for group-resource %v", info.GroupResource)
+	}
+	if err != nil {
+		return schema.GroupVersionResource{}, fmt.Errorf("unable to find preferred version to list matching resource names: %v", err)
+	}
+
+	return fullResources[0], nil
+}
+
+// ReferenceFor returns a new ObjectReference for the given group-resource and name.
+// The group-resource is converted into a group-version-kind using the given RESTMapper.
+// You can use this to easily construct an object reference for use in the DescribedObject
+// field of CustomMetricInfo.
+func ReferenceFor(mapper apimeta.RESTMapper, name types.NamespacedName, info provider.CustomMetricInfo) (custom_metrics.ObjectReference, error) {
+	kind, err := mapper.KindFor(info.GroupResource.WithVersion(""))
+	if err != nil {
+		return custom_metrics.ObjectReference{}, err
+	}
+
+	// NB: return straight value, not a reference, so that the object can easily
+	// be copied for use multiple times with a different name.
+	return custom_metrics.ObjectReference{
+		APIVersion: kind.Group + "/" + kind.Version,
+		Kind:       kind.Kind,
+		Name:       name.Name,
+		Namespace:  name.Namespace,
+	}, nil
+}
+
+// ListObjectNames uses the given dynamic client to list the names of all objects
+// of the given resource matching the given selector.  Namespace may be empty
+// if the metric is for a root-scoped resource.
+func ListObjectNames(mapper apimeta.RESTMapper, client dynamic.Interface, namespace string, selector labels.Selector, info provider.CustomMetricInfo) ([]string, error) {
+	res, err := ResourceFor(mapper, info)
+	if err != nil {
+		return nil, err
+	}
+
+	var resClient dynamic.ResourceInterface
+	if info.Namespaced {
+		resClient = client.Resource(res).Namespace(namespace)
+	} else {
+		resClient = client.Resource(res)
+	}
+
+	matchingObjectsRaw, err := resClient.List(metav1.ListOptions{LabelSelector: selector.String()})
+	if err != nil {
+		return nil, err
+	}
+
+	if !apimeta.IsListType(matchingObjectsRaw) {
+		return nil, fmt.Errorf("result of label selector list operation was not a list")
+	}
+
+	var names []string
+	err = apimeta.EachListItem(matchingObjectsRaw, func(item runtime.Object) error {
+		objName := item.(*unstructured.Unstructured).GetName()
+		names = append(names, objName)
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	return names, nil
+}