Move activator count computation into KPA

[markusthoemmes]

Fixes #11373

Proposed Changes

See the above issue for further infromation. Changes the default to 2 rather than all, which should be correct in most cases anyway as the autoscaler would immediately set it to 2 in most cases anyway.

Release Note

Fixed a bug where traffic would briefly be routed 'wrong', leading to errors due to exceeded queues in deployments with a large activator count and a low service pod count.

/assign @julz @vagababov

[markusthoemmes]

/hold

For a bit of discussion I just posted #11373 (comment)

[codecov]

Codecov Report

Merging #11375 (2e946a5) into main (1b2b759) will increase coverage by 0.02%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main   #11375      +/-   ##
==========================================
+ Coverage   87.68%   87.71%   +0.02%     
==========================================
  Files         191      191              
  Lines        9219     9224       +5     
==========================================
+ Hits         8084     8091       +7     
+ Misses        880      879       -1     
+ Partials      255      254       -1     

Impacted Files	Coverage Δ
pkg/autoscaler/scaling/multiscaler.go	89.09% <ø> (+1.48%)	⬆️
pkg/reconciler/autoscaling/kpa/kpa.go	95.04% <85.71%> (+0.30%)	⬆️
pkg/autoscaler/scaling/autoscaler.go	97.77% <100.00%> (-0.11%)	⬇️
pkg/reconciler/domainmapping/reconciler.go	91.54% <0.00%> (-0.99%)	⬇️
pkg/reconciler/configuration/configuration.go	88.28% <0.00%> (+1.56%)	⬆️

---

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[vagababov]

/lgtm

[markusthoemmes]

/unhold

[julz]

pretty much looks great to me! I might well be wrong, but if Im reading kpa_test right I think we're missing coverage for the two cases (0, -1) where TargetBurstCapacity isn't a positive number, (if so) worth adding those?

[markusthoemmes]

Thanks @julz. Added a targeted unit test just for the computation function to make the cases a bit clearer.

[julz]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: julz

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