HPA: only send updates when the status has changed

[DirectXMan12]

This commit only sends updates if the status has actually changed.
Since the HPA runs at a regular interval, this should reduce the volume
of writes, especially on short HPA intervals with relatively constant
metrics.

Fixes #47077

Release note:

The HorizontalPodAutoscaler controller will now only send updates when it has new status information, reducing the number of writes caused by the controller.

[DirectXMan12]

cc @kubernetes/sig-autoscaling-pr-reviews @derekwaynecarr @liggitt @dak1n1

[derekwaynecarr]

To elaborate, the writes cause other side effects (like frequent snapshotting of etcd), which can have other side effects when trying to run and restore HA etcd deployments. Minimizing the write frequency is a priority.

[luxas]

Does this bugfix target v1.7?

[fgrzadkowski]

@DirectXMan12 Aren't we always updating observedGeneration?

[derekwaynecarr]

@luxas -- yes, i updated milestone.

[DirectXMan12]

@DirectXMan12 Aren't we always updating observedGeneration?

We actually never update observed generation, AFAICT.

[derekwaynecarr]

/lgtm

with this change, what amount of traffic should i expect per HPA in a steady state cluster environment?

[derekwaynecarr]

@k8s-bot pull-kubernetes-e2e-gce-etcd3 test this

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: DirectXMan12, derekwaynecarr

Associated issue: 47077

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[k8s-github-robot]

@k8s-bot test this [submit-queue is verifying that this PR is safe to merge]

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 46979, 47078, 47138, 46916)

[k8s-ci-robot]

@DirectXMan12: The following tests failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-unit	00a19f1	link	@k8s-bot pull-kubernetes-unit test this
pull-kubernetes-kubemark-e2e-gce	00a19f1	link	@k8s-bot pull-kubernetes-kubemark-e2e-gce test this

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