fix: add rule label to node_readiness_evaluation_duration_seconds metric

[Shreya2005-2005]

Description

node_readiness_evaluation_duration_seconds was defined as a plain
Histogram with no labels. All other per-rule metrics include a rule
label. Without it, operators cannot identify which rule is slow in
multi-rule clusters.

Related Issue

Fixes #243

Type of Change

/kind bug

Testing

• go build ./... passes
• make lint passes (0 issues)
• go test ./internal/webhook/... passes (25/25)

Checklist

• make test passes
• make lint passes

Does this PR introduce a user-facing change?

NONE

[netlify]

✅ Deploy Preview for node-readiness-controller canceled.

Name	Link
🔨 Latest commit	a063a76
🔍 Latest deploy log	https://app.netlify.com/projects/node-readiness-controller/deploys/6a0414cfe0ba1a0007026b3b

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Shreya2005-2005
Once this PR has been reviewed and has the lgtm label, please assign tallclair for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

Hi @Shreya2005-2005. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.