[client-go] Polymorphic Scale Client

This introduces a polymorphic scale client capable of operating against
scale subresources which return different group-versions of Scale.  The
scale subresources may be in group-versions different than the scale
itself, so that we no longer need a copy of every scalable resource in
the extensions API group.

To discovery which Scale group-versions go to which subresources,
discovery is used.

The scale client maintains its own internal versions and conversions to
several external versions, with a "hub" version that's a copy of the
autoscaling internal version.

It currently supports the following group-versions for Scale subresources:

- extensions/v1beta1.Scale
- autoscaling/v1.Scale

Kubernetes-commit: d61a2d90372c301dd11088df8941acf2bb01c38c

scale/scheme/types.go

@@ -0,0 +1,60 @@
+/*
+Copyright 2017 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 scheme
+
+import (
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+// This file contains our own "internal" version of scale that we use for conversions,
+// since we can't use the main Kubernetes internal versions.
+
+// +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
+
+// Scale represents a scaling request for a resource.
+type Scale struct {
+	metav1.TypeMeta
+	// Standard object metadata; More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#metadata.
+	// +optional
+	metav1.ObjectMeta
+
+	// defines the behavior of the scale. More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#spec-and-status.
+	// +optional
+	Spec ScaleSpec
+
+	// current status of the scale. More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#spec-and-status. Read-only.
+	// +optional
+	Status ScaleStatus
+}
+
+// ScaleSpec describes the attributes of a scale subresource.
+type ScaleSpec struct {
+	// desired number of instances for the scaled object.
+	// +optional
+	Replicas int32
+}
+
+// ScaleStatus represents the current status of a scale subresource.
+type ScaleStatus struct {
+	// actual number of observed instances of the scaled object.
+	Replicas int32
+
+	// label query over pods that should match the replicas count.
+	// More info: https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#label-selectors
+	// +optional
+	Selector *metav1.LabelSelector
+}