feat(devops): Build Prometheus Metrics Middleware and Grafana Dashboard for FastAPI Backend Monitoring

[advikdivekar]

Summary

Fixes #636

This PR integrates Prometheus telemetry into the FastAPI backend and provides a ready-to-import Grafana dashboard for production visibility.

Changes

• backend/middleware/metrics.py — Prometheus Integration

  • Uses prometheus-fastapi-instrumentator to auto-instrument all HTTP routes
  • Registers custom application-level metrics:
    • helpdesk_ai_analyze_total (Counter) — AI requests by model version & status
    • helpdesk_ai_analyze_duration_seconds (Histogram) — AI latency with p50/p95 buckets
    • helpdesk_active_tickets_total (Gauge) — live active ticket count
    • helpdesk_duplicate_detections_total (Counter) — duplicate flagging rate
    • helpdesk_ocr_processing_total (Counter) — OCR call count by status
  • Secure /metrics endpoint: Bearer token (METRICS_SECRET_TOKEN) or IP allowlist (METRICS_ALLOWED_IPS) — returns 403 otherwise
  • Graceful degradation when prometheus-fastapi-instrumentator is not installed

• deploy/prometheus/prometheus.yml — Prometheus scrape config targeting backend:8000 with token auth and a node_exporter sidecar for system metrics

• deploy/grafana/dashboard.json — Grafana dashboard with 9 panels:

  1. HTTP Request Rate by Endpoint
  2. Request Latency p95 (gauge)
  3. AI Analysis Requests by Model & Status
  4. AI Analysis Latency p50/p95
  5. Active Tickets (stat)
  6. Duplicate Detections per 10m (stat)
  7. Memory Usage GiB (stat)
  8. CPU Usage (stat)
  9. Error Rate 4xx/5xx by Endpoint

• backend/requirements.txt — adds prometheus-fastapi-instrumentator>=6.1.0, prometheus-client>=0.19.0, pycryptodome>=3.20.0

Test Plan

• Import setup_metrics(app) in backend/main.py and start the server
• GET /metrics with no token → 403
• GET /metrics with correct Authorization: Bearer <token> → 200 Prometheus text
• Import deploy/grafana/dashboard.json into a Grafana instance pointing at Prometheus
• All 9 panels render without errors

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[advikdivekar]

@ritesh-1918 please review my PR, thank you