feat: expose Prometheus scrape endpoint at /metrics/prometheus

In-memory MetricsCollector is preserved for agent-level stats.
prometheus-fastapi-instrumentator adds standard HTTP metrics
(request count, latency histograms) for Grafana/Alertmanager.

app/api/main.py

@@ -5,12 +5,14 @@ from slowapi import _rate_limit_exceeded_handler
 from slowapi.errors import RateLimitExceeded
 from app.api.limiter import limiter
 import os
+from prometheus_fastapi_instrumentator import Instrumentator
 
 app = FastAPI(
     title="AI Code Review Agent",
     description="Multi-agent AI platform that simulates a software engineering team",
     version="0.1.0"
 )
+Instrumentator().instrument(app).expose(app, endpoint="/metrics/prometheus")
 
 app.state.limiter = limiter
 app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)

requirements.txt

@@ -25,4 +25,5 @@ slowapi
 pytest-asyncio
 fastapi==0.115.12
 uvicorn==0.34.3
-tenacity==8.3.0
+tenacity==8.3.0
+prometheus-fastapi-instrumentator