[coredns] [coredns] Add coredns_health_request_failures_total metric

[mahipdeora25]

Generated by FBO — DRAFT for human review

Closes FRAGENT-3502

Reality-check reasoning

In metrics.py, the health plugin section under DEFAULT_METRICS only maps coredns_health_request_duration_seconds → health_request_duration. There is no entry for coredns_health_request_failures_total in either DEFAULT_METRICS or NEW_METRICS. The metadata.csv (119 metrics) similarly contains no health_request_failures metric — only health-related entries visible are for request duration. The requested metric is clearly absent from the current codebase and would need to be added to both metrics.py and metadata.csv.

This PR was opened as a Draft. A human should review the diff,
re-run validators if needed, and mark Ready for Review.

[dd-octo-sts]

Validation Report

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metadata	Validate metadata.csv metric definitions	✅
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[datadog-prod-us1-3]

   

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⚠️ Warnings

🚦 5 Pipeline jobs failed

PR | test / test (linux, ubuntu-22.04, coredns, CoreDNS (py3.13-1.8), py3.13-1.8) / CoreDNS (py3.13-1.8)-py3.13-1.8     

🔧 Fix in code (Fix with Cursor).

3 failed tests due to assertion errors on missing metrics: 'coredns.health_request_failures.count' in metadata and unasserted collected metrics.

PR | test / test-minimum-base-package (linux, ubuntu-22.04, coredns, CoreDNS (py3.13-1.8), py3.13-1.8) / minimum-base-package-CoreDNS (py3.13-1.8)-py3.13-1.8     

🔧 Fix in code (Fix with Cursor).

3 failed tests. AssertionError: Metadata assertion errors using metadata.csv: Expect `coredns.health_request_failures.count` to be in metadata.csv.

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Package "coredns" has changes that require a changelog. Please run `ddev release changelog new` to add it.

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ℹ️ Info

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🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 94.41% (+7.03%)

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