feat(api): Add Prometheus metrics endpoint (issue #219)

[Richardkingz2019]

Summary

Closes #219

Adds a Prometheus-compatible GET /metrics endpoint to the NestJS API, providing full production observability.

Changes

New files

• api/src/metrics/metrics.service.ts — MetricsService wraps a dedicated prom-client Registry and exposes:
  • http_requests_total (Counter, labels: method/path/status_code)
  • http_request_duration_seconds (Histogram, 10-bucket, same labels)
  • websocket_connections_total (Counter)
  • websocket_active_connections (Gauge)
  • Node.js default metrics via collectDefaultMetrics
• api/src/metrics/metrics.controller.ts — GET /metrics returns Prometheus text format with correct Content-Type
• api/src/metrics/metrics.module.ts — NestJS module exporting MetricsService
• api/src/metrics/metrics.service.spec.ts — unit tests for all counters, histogram, gauge, and default metrics
• api/src/metrics/metrics.controller.spec.ts — unit tests for response headers and body

Modified files

• api/package.json — add prom-client@15.1.3, @nestjs/testing@10.3.0 (devDep)
• api/src/app.module.ts — import MetricsModule
• api/src/gateways/gateways.module.ts — import MetricsModule so StreamsGateway can inject MetricsService
• api/src/gateways/streams.gateway.ts — inject MetricsService (optional) and track WS connect/disconnect
• api/src/middleware/request-logger.middleware.ts — inject MetricsService (optional) and record HTTP counters/histogram on response finish

Acceptance criteria

Criterion	Status
GET /metrics returns Prometheus text format	✅
Default Node.js process metrics included	✅
HTTP request counter incremented per route	✅
HTTP request duration histogram populated	✅
WebSocket connection counter tracked	✅

Testing

cd api && npm test   # 21 tests, 4 suites — all pass
cd api && npx tsc --noEmit   # exit 0
cd api && npm run lint        # exit 0

[Xhristin3]

Love the clean separation — metrics module isolated from app code, /metrics excluded from request logging so we don't pollute our own data, and tests cover gauges + counters. LGTM, merging 🎉