HPA metrics specificity improvements

[damemi]

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: kubernetes/community#2055

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

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

[DirectXMan12]

/ok-to-test

[DirectXMan12]

For the APIServices int test, you need to enable the API version in PKG/master/master.go

[DirectXMan12]

Make sure we have validation on the metric target

[DirectXMan12]

you missed some cases of switching to scheme.ParameterCodec in the custom metrics client.

[DirectXMan12]

conversions aren't quite right -- you don't properly populate the type field in the metric target, it looks like

[DirectXMan12]

why are these nil? If they're not needed, don't implement them

[damemi]

That's what I thought, but without them defined as nil they get generated for some reason:

# k8s.io/kubernetes/pkg/apis/autoscaling/v2beta1
pkg/apis/autoscaling/v2beta1/zz_generated.conversion.go:507:12: undefined: Convert_v2beta1_CrossVersionObjectReference_To_autoscaling_MetricTarget
pkg/apis/autoscaling/v2beta1/zz_generated.conversion.go:519:12: undefined: Convert_autoscaling_MetricTarget_To_v2beta1_CrossVersionObjectReference
!!! [0530 21:24:29] Call tree:
!!! [0530 21:24:29]  1: /home/mdame/go/src/k8s.io/kubernetes/hack/lib/golang.sh:595 kube::golang::build_binaries_for_platform(...)
!!! [0530 21:24:29]  2: hack/make-rules/build.sh:27 kube::golang::build_binaries(...)
!!! [0530 21:24:29] Call tree:
!!! [0530 21:24:29]  1: hack/make-rules/build.sh:27 kube::golang::build_binaries(...)
!!! [0530 21:24:29] Call tree:
!!! [0530 21:24:29]  1: hack/make-rules/build.sh:27 kube::golang::build_binaries(...)
make: *** [Makefile:92: all] Error 1

(Output from just running make with those functions removed and zz_generated.conversion deleted)

I think it has to do with the fact that Target in v2beta1 is a CrossVersionObjectReference, but in v2beta2 we renamed that to DescribedObject and created a new field called Target that is a MetricTarget. So it's seeing Target=Target and trying to auto-convert CrossVersionObjectReference->MetricTarget and telling me I need to manually convert it (which I can't)

If I'm wrong, or there's a way around that, I can make the change (or I can add a comment explaining these functions, which I probably should have done first)

[smarterclayton]

/approve

[DirectXMan12]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: damemi, DirectXMan12, smarterclayton

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• api/OWNERS [smarterclayton]
• cmd/kube-apiserver/OWNERS [smarterclayton]
• docs/api-reference/OWNERS [smarterclayton]
• hack/OWNERS [smarterclayton]
• pkg/OWNERS [smarterclayton]
• staging/OWNERS [smarterclayton]
• test/e2e/instrumentation/OWNERS [DirectXMan12,smarterclayton]
• test/integration/dryrun/OWNERS [smarterclayton]
• test/integration/etcd/OWNERS [smarterclayton]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-github-robot]

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 here.