Add a /api/metrics protected by METRICS_AUTH_TOKEN with prom-client

[greatest0fallt1me]

Description

This is a backend issue for the GrantFox campaign. Expose /api/metrics protected by a bearer token specified via env, scraped by Prometheus.

Requirements and Context

• Token required header
• prom-client registry
• Default Node metrics
• Custom counters/histograms
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b task/metrics-auth

2. Implement changes
   • src/routes/metrics.ts (new)
   • src/metrics/registry.ts (new)
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

feat: token-protected /api/metrics

Acceptance Criteria

• Auth required
• Defaults exposed
• Custom metrics
• Doc updated

Guidelines

• Minimum 90% test coverage on changed lines
• Input validation at the boundary; standardized error envelope
• Structured logging with correlation IDs
• Clear documentation and inline comments
• Timeframe: 96 hours