[wip] Make autoscaler instrument async so we can remove metric attributes when revisions go away

[dprotaso]

Fixes https://github.com/knative/serving/issues/16200

Proposed Changes

• This switches instruments to async allow us to remove them when revisions go away

Release Note

Switch to async metric instrumentation to avoid unbounded memory growth

[codecov[bot]]

Codecov Report

:x: Patch coverage is 94.07407% with 8 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 80.06%. Comparing base (5fbd94e) to head (41c9473). :warning: Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/autoscaler/scaling/metrics.go	94.02%	4 Missing :warning:
pkg/autoscaler/scaling/autoscaler.go	80.00%	2 Missing :warning:
pkg/reconciler/autoscaling/kpa/metrics.go	93.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16300      +/-   ##
==========================================
- Coverage   80.10%   80.06%   -0.04%     
==========================================
  Files         215      215              
  Lines       13332    13361      +29     
==========================================
+ Hits        10679    10697      +18     
- Misses       2294     2305      +11     
  Partials      359      359              

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[dprotaso]

Ok tested this locally and confirmed the metrics go away when the revisions are deleted

/assign @Cali0707 @linkvt

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Cali0707, dprotaso

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[dprotaso]

/cherry-pick release-1.19 /cherry-pick release-1.20

[knative-prow-robot]

@dprotaso: new pull request created: #16301

In response to this:

/cherry-pick release-1.19 /cherry-pick release-1.20

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[knative-prow-robot]

@dprotaso: new pull request created: #16302

In response to this:

/cherry-pick release-1.19 /cherry-pick release-1.20

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.