Fixes an activator panic when the throttle encounters a cache.DeleteFinalStateUnknown

[dprotaso]

Fixes #12676

Note: I couldn't figure out a way to reproduce this with the client-go fakes

Release Note

Fix an activator crash that could disrupt traffic (503)

[dprotaso]

/cherry-pick release-1.2

[knative-prow-robot]

@dprotaso: once the present PR merges, I will cherry-pick it on top of release-1.2 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.2

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[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso

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[dprotaso]

flaky - #12490

/retest

[codecov]

Codecov Report

Merging #12679 (8737b65) into main (d48569e) will increase coverage by 0.04%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main   #12679      +/-   ##
==========================================
+ Coverage   87.27%   87.31%   +0.04%     
==========================================
  Files         196      196              
  Lines        9736     9739       +3     
==========================================
+ Hits         8497     8504       +7     
+ Misses        952      950       -2     
+ Partials      287      285       -2     

Impacted Files	Coverage Δ
pkg/activator/net/throttler.go	87.95% <40.00%> (-0.90%)	⬇️
pkg/reconciler/revision/background.go	91.81% <0.00%> (+1.81%)	⬆️
pkg/autoscaler/statforwarder/processor.go	94.44% <0.00%> (+5.55%)	⬆️
pkg/autoscaler/statforwarder/forwarder.go	96.29% <0.00%> (+5.55%)	⬆️

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[dprotaso]

/retest

[nader-ziada]

/lgtm

[knative-prow-robot]

@dprotaso: new pull request created: #12680

In response to this:

/cherry-pick release-1.2

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/test-infra repository.