✨ Add admission response status code metrics for webhooks

[mattsu2020]

What does this do, and why do we need it?

This PR adds an admission-response-level metric:

controller_runtime_webhook_admission_responses_total{webhook,allowed,admission_code}

This PR intentionally does not change the HTTP response status for valid AdmissionReview responses. Kubernetes admission webhooks are expected to return HTTP 200 with an AdmissionReview body, including when an admission decision is denied.

The existing controller_runtime_webhook_requests_total{code=...} metric continues to represent the HTTP transport status code observed by promhttp. The new metric separately exposes AdmissionResponse.status.code, making admission decisions and errors such as 400, 403, 429, and 500 observable without changing the existing HTTP request metric semantics.

The common response series are initialized when an Admission webhook is registered directly with the webhook server or configured through StandaloneWebhook. Responses with an unset status code follow the existing Response.Complete behavior and are recorded as 200.

Validation

• go test ./pkg/webhook/...
• go vet ./pkg/webhook/...

Addresses the metrics aspect of #2972.

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mattsu2020
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[mattsu2020]

Hi @vincepri @troy0820,
Could you please take a look at this PR when you have a chance?

Thanks!