Merge pull request #64097 from damemi/hpa-metrics-specificity

Automatic merge from submit-queue (batch tested with PRs 67894, 64097). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

HPA metrics specificity improvements

**What this PR does / why we need it**:
Improves available specificity for HPA metrics by adding metric selector fields for metrics of Pods and Objects.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Implements this KEP: https://github.com/kubernetes/community/pull/2055

**Special notes for your reviewer**:
Need to add/update tests?

**Release note**:

```release-note
Introduces autoscaling/v2beta2 and custom_metrics/v1beta2, which implement metric selectors for Object and Pods metrics, as well as allowing AverageValue targets on Objects, similar to External metrics.
```

/assign @DirectXMan12

Kubernetes-commit: fdb5707194d56cbbd0da11c5be3a2a5653e714c9

vendor/k8s.io/api/autoscaling/v2beta2/doc.go

@@ -0,0 +1,20 @@
+/*
+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.
+*/
+
+// +k8s:deepcopy-gen=package
+// +k8s:openapi-gen=true
+
+package v2beta2