admission: make admission metrics compositional

[sttts]

Metrics emission of admission plugins and the admission chain can be implemented compositionally, i.e. completely independently from the chain logic. This PR does that, moves the whole metrics code into a sub-package to contain complexity. The plumbing logic for the emitted metrics finally is cleanly done in the apiserver bootstrapping code, instead of being totally interleaved with the core admission logic.

Ratio:

• considerably less complexity
• admission plugins are compositional, including the chain. We cannot assume that there is only one chain at the outside of the admission plugin structure. Downstream projects might have more complex admission chains, i.e. multiple chain object nested.
• addition of metrics is plumbing and should be in the apiserver plumbing code. This makes it much easier to reason about the security critical admission chain.

Follow-up of #55183 and based on #55919.

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@sttts: Adding do-not-merge/release-note-label-needed because the release note process has not been followed.

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[k8s-ci-robot]

@sttts: Adding do-not-merge/release-note-label-needed because the release note process has not been followed.

One of the following labels is required "release-note", "release-note-action-required", or "release-note-none".
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[k8s-ci-robot]

@sttts: Adding do-not-merge/release-note-label-needed because the release note process has not been followed.

One of the following labels is required "release-note", "release-note-action-required", or "release-note-none".
Please see: https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md#write-release-notes-if-needed.

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[hzxuzhonghu]

Nice wrap.

[k8s-ci-robot]

@sttts: Adding do-not-merge/release-note-label-needed because the release note process has not been followed.

One of the following labels is required "release-note", "release-note-action-required", or "release-note-none".
Please see: https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md#write-release-notes-if-needed.

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[sttts]

@jpbetz if you can take a look, based on your composition PR.

[sttts]

/retest

[jpbetz]

Thanks @sttts

/lgtm

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deads2k, jpbetz, sttts

Associated issue: 55183

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Needs approval from an approver in each of these OWNERS Files:

• plugin/pkg/admission/OWNERS [deads2k]
• staging/src/k8s.io/apiextensions-apiserver/OWNERS [deads2k,sttts]
• staging/src/k8s.io/apiserver/OWNERS [deads2k,sttts]
• staging/src/k8s.io/kube-aggregator/OWNERS [deads2k]
• staging/src/k8s.io/sample-apiserver/OWNERS [deads2k,sttts]

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[sttts]

/assign status/approved-for-milestone

[k8s-github-robot]

[MILESTONENOTIFIER] Milestone Pull Request Current

@deads2k @jpbetz @sttts

Note: If this pull request is not resolved or labeled as priority/critical-urgent by Wed, Nov 22 it will be moved out of the v1.9 milestone.

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• kind/cleanup: Adding tests, refactoring, fixing old bugs.

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[k8s-github-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 55938, 56055, 53385, 55796, 55922). If you want to cherry-pick this change to another branch, please follow the instructions here.

[k8s-ci-robot]

@sttts: The following tests failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-unit	baba0c8	link	/test pull-kubernetes-unit
pull-kubernetes-node-e2e	baba0c8	link	/test pull-kubernetes-node-e2e
pull-kubernetes-verify	baba0c8	link	/test pull-kubernetes-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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