use the /scale subresource to when updating replica count

[dprotaso]

This replaces #16419

Fixes #15528

Some notable differences

• don't use the scale client Get since that is an API call - we don't want to do that for every scale call.
  • use the existing PodScalable resource which has the currentScale
• Drop spec.template from PodScalable since we don't need it to scale the resource
  • This saves memory in the autoscaler and controller
• This has the added benefit of not having to DeepCopy and computing a
  JSONPatch via diffing two json byte strings which would take about 400ms

For now we expected the spec.replicas to be the patch required to update the replica count.

Note: we don't use the scale client to Get the resource prior since it will hit the API server

Release Note

Autoscaler now supports CRDs that implement the `/scale` subresource for updating

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.23%. Comparing base (8badc85) to head (0f85a06).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16540      +/-   ##
==========================================
+ Coverage   80.15%   80.23%   +0.08%     
==========================================
  Files         217      217              
  Lines       13547    13532      -15     
==========================================
- Hits        10859    10858       -1     
+ Misses       2319     2310       -9     
+ Partials      369      364       -5     

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[dprotaso]

/assign @linkvt

[dsimansk]

@dprotaso I assume, this is intentionally wide to allow for scaling different resourecs like mentioned in the issue CloneSet, AppSet. Correct?

[dprotaso]

Yeah that's the intent

[dsimansk]

/lgtm

For other reviewers
/hold

[linkvt]

Nice, clean change - didn't expect it to be that simple!

Also didn't know that the JSON Patch add operation is more forgiving than replace 💡

/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, linkvt

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[dprotaso]

/hold cancel

[chenzhiguo]

👍