feat: add enforcement latency metric with operation labels

[rawadhossain]

Description

• Adds node_readiness_enforcement_latency_seconds with add/remove operation labels, keeping node_readiness_reconciliation_latency_seconds unchanged.
• Updates scale PromQL queries and the scalability report.
• Updates monitoring.md and docs/TEST_README.md.
• Adds related tests.

Related to #446

Type of Change

/kind feature

Testing

make test, make lint, go vet, gofmt -l all pass.

Checklist

• make test passes
• make lint passes

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: rawadhossain
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[kubernetes-prow]

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