More assertive borrowing by exempt

[MikeSpreitzer]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This PR changes the way that exempt priority levels borrow from non-exempt ones in API Priority and Fairness. Note that the code does not match the KEP on this matter, which is Issue #124125 . This PR shifts the code to a middle position. A companion PR (kubernetes/enhancements#4623) shifts the KEP to the same place.

Which issue(s) this PR fixes:

This is the code part of one possible way to addressing #124125 .

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

- [KEP]: https://github.com/kubernetes/enhancements/tree/master/keps/sig-api-machinery/1040-priority-and-fairness

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MikeSpreitzer

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• staging/src/k8s.io/apiserver/pkg/util/flowcontrol/OWNERS [MikeSpreitzer]

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[MikeSpreitzer]

/cc @tkashem
/cc @wojtek-t
/uncc @yue9944882

[wojtek-t]

This change makes sense to me, but we need some tests for it...

[fedebongio]

/triage accepted

[MikeSpreitzer]

/retest

[MikeSpreitzer]

Here is an example of running the new unit test:

mspreitz@mjs13 flowcontrol % go test . -v -run TestUpdateBorrowing
=== RUN   TestUpdateBorrowing
    exempt_borrowing_test.go:82: Scenario 1: expected and got 970 for exempt
    exempt_borrowing_test.go:87: Scenario 1: expected and got 120 for hi
    exempt_borrowing_test.go:92: Scenario 1: expected and got 60 for mid
    exempt_borrowing_test.go:97: Scenario 1: expected and got 30 for lo
    exempt_borrowing_test.go:113: Scenario 2: expected and got 330 for exempt
    exempt_borrowing_test.go:118: Scenario 2: expected and got 180 for hi
    exempt_borrowing_test.go:123: Scenario 2: expected and got 330 for mid
    exempt_borrowing_test.go:128: Scenario 2: expected and got 30 for lo
    exempt_borrowing_test.go:146: Scenario 3: expected and got 540 for exempt
    exempt_borrowing_test.go:151: Scenario 3: expected and got 240 for hi
    exempt_borrowing_test.go:156: Scenario 3: expected and got 60 for mid
    exempt_borrowing_test.go:161: Scenario 3: expected and got 30 for lo
--- PASS: TestUpdateBorrowing (0.00s)
PASS
ok  	k8s.io/apiserver/pkg/util/flowcontrol	(cached)