Add operations runbook for metrics subsystem

[afeezorobsco-cyber]

##close #770

Summary

This PR adds an operations runbook for the metrics subsystem, documenting configuration, architecture, common failure scenarios, alerting guidance, and recovery procedures based on the current implementation.

Changes

• Added docs/runbook-metrics.md.
• Documented metrics subsystem configuration and environment variables.
• Documented how metrics are collected and exposed.
• Added operational guidance for common failure scenarios.
• Added recovery procedures for diagnosing and restoring metrics collection.
• Cross-referenced the relevant routes, services, and configuration files.

Verification

The runbook was validated against the current implementation by reviewing the relevant metrics routes, services, middleware, and configuration.

Checks

• ✅ npm run lint
• ✅ npm test
• ✅ npm run build

Notes

This is a documentation-only change. No production code or application behaviour has been modified.

Closes #

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Local Test Output

Paste the full output of:

npm run lint
npm test
npm run build

before submitting the PR.

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[afeezorobsco-cyber]

@mikewheeleer pls review and merge

[mikewheeleer]

appreciate this — thanks for picking it up 🙌