Add RPS support for HPA

[yanweiguo]

Fixes #5216

Proposed Changes

• Allow HPA to use RPS metric as annotation.
• Add RPS support for HPA.
• The MetricProvider change was done in Report RPS for autoscaler metrics #5238.

Release Note

Autoscaling using HPA supports requests-per-second as metric. 

[knative-prow-robot]

@yanweiguo: 0 warnings.

In response to this:

Fixes #5216

Proposed Changes

• Allow HPA to use RPS metric as annotation.
• Add RPS support for HPA.
• The MetricProvider change was done in Report RPS for autoscaler metrics #5238.

Release Note

Autoscaling using HPA supports requests-per-second as metric. 

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[vagababov]

I presume Integration test comes next?
/lgtm
/approve
/hold

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vagababov, yanweiguo

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[yanweiguo]

I presume Integration test comes next?
/lgtm
/approve
/hold

Yeah, will add to #5244 as well

[yanweiguo]

/hold cancel

[yanweiguo]

/test pull-knative-serving-integration-tests

[knative-test-reporter-robot]

The following jobs failed due to test flakiness:

Test name	Triggers	Retries
pull-knative-serving-integration-tests	pull-knative-serving-integration-tests pull-knative-serving-integration-tests	2/3

Automatically retrying...
/test pull-knative-serving-integration-tests

[vagababov]

/lgtm

[yanweiguo]

/test pull-knative-serving-unit-tests