Add allowed/denied metrics for authorizers

[liggitt]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Records metrics for terminal decisions by all configured authorizers.

Which issue(s) this PR fixes:

Fixes #123324

Does this PR introduce a user-facing change?

kube-apiserver now reports metrics for authorization decisions in the `apiserver_authorization_decisions_total` metric, labeled by authorizer type, name, and decision.

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

- [KEP]: https://github.com/kubernetes/enhancements/tree/master/keps/sig-auth/3221-structured-authorization-configuration

/sig auth
/assign @ritazh @palnabarun @enj

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt

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Needs approval from an approver in each of these files:

• pkg/kubeapiserver/authorizer/OWNERS [liggitt]
• staging/src/k8s.io/apiserver/pkg/authorization/OWNERS [liggitt]
• staging/src/k8s.io/apiserver/pkg/server/options/OWNERS [liggitt]
• test/OWNERS [liggitt]
• vendor/OWNERS [liggitt]

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[liggitt]

/retest

[ping035627]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: eedb23ff2ffb4b1eb208dbf4f65c5504472eae5c

[alexzielenski]

/triage accepted