Kpa support scale subresource

[thiagomedina]

Fixes #15528

Proposed Changes

• Use the Kubernetes /scale subresource to read and patch replica count

Release Note

[knative-prow]

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[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.18%. Comparing base (0c341d4) to head (7378056).

Files with missing lines	Patch %	Lines
pkg/reconciler/autoscaling/kpa/scaler.go	71.42%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16419      +/-   ##
==========================================
- Coverage   80.20%   80.18%   -0.02%     
==========================================
  Files         217      217              
  Lines       13511    13506       -5     
==========================================
- Hits        10836    10830       -6     
- Misses       2310     2311       +1     
  Partials      365      365              

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[dprotaso]

Hey @thiagomedina - I've poked at the scale subresource before. One thing I didn't see in my prior investigation was whether you could create informers that return the autoscaling.v1.Scale object.

Curious if you've played around with that? Then the autoscaler informers wouldn't need the entire deployment in memory.