feat: expose Prometheus metrics endpoint

backend/app/main.py

@@ -19,6 +19,7 @@ from slowapi.middleware import SlowAPIMiddleware
 from app.config import get_settings
 from app.rate_limit import limiter
 from app.database import init_db, get_db
+from app.observability import setup_prometheus_metrics
 from app.rag.vectorstore import get_chroma_client
 from app.scheduler import start_scheduler, stop_scheduler
 
@@ -170,6 +171,8 @@ app.include_router(chat_router, prefix="/api/v1")
 app.include_router(github_router, prefix="/api/v1")
 app.include_router(admin_router, prefix="/api/v1")
 
+setup_prometheus_metrics(app)
+
 
 # ── Health Check ─────────────────────────────────────
 @app.get("/api/health")

backend/app/observability.py

@@ -0,0 +1,46 @@
+"""Prometheus instrumentation for the FastAPI application."""
+
+import sys
+
+try:
+    import resource
+except ImportError:  # pragma: no cover - resource is unavailable on some platforms.
+    resource = None
+
+from fastapi import FastAPI
+from prometheus_client import Gauge
+from prometheus_fastapi_instrumentator import Instrumentator
+
+APP_PROCESS_RSS_BYTES = Gauge(
+    "app_process_resident_memory_bytes",
+    "Resident memory used by the backend process in bytes.",
+)
+
+
+def _get_process_rss_bytes() -> float:
+    if resource is None:
+        return 0.0
+
+    usage = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss
+    if sys.platform == "darwin":
+        return float(usage)
+    return float(usage * 1024)
+
+
+APP_PROCESS_RSS_BYTES.set_function(_get_process_rss_bytes)
+
+
+def setup_prometheus_metrics(app: FastAPI) -> Instrumentator:
+    """Expose process and HTTP metrics on ``/metrics`` for Prometheus."""
+    instrumentator = Instrumentator(
+        should_group_status_codes=True,
+        should_ignore_untemplated=True,
+        excluded_handlers=["/metrics"],
+    )
+    instrumentator.instrument(app).expose(
+        app,
+        endpoint="/metrics",
+        include_in_schema=False,
+    )
+    app.state.prometheus_instrumentator = instrumentator
+    return instrumentator

backend/requirements.txt

@@ -55,6 +55,7 @@ huggingface-hub
 # Production
 gunicorn
 slowapi
+prometheus-fastapi-instrumentator
 
 # File Validation
 #sudo apt-get install libmagic1 // for Debian/Ubuntu

backend/tests/test_observability.py

@@ -0,0 +1,13 @@
+def test_metrics_endpoint_exposes_prometheus_payload(client):
+    client.get("/api/health")
+
+    response = client.get("/metrics")
+
+    assert response.status_code == 200
+    assert response.headers["content-type"].startswith("text/plain")
+
+    body = response.text
+    assert "python_info" in body
+    assert "app_process_resident_memory_bytes" in body
+    assert "http_requests_total" in body
+    assert "/api/health" in body