feat(prometheus): add custom metrics

7project/backend/app/services/prometheus.py

@@ -0,0 +1,48 @@
+from typing import Callable
+from prometheus_fastapi_instrumentator.metrics import Info
+from prometheus_client import Gauge
+from sqlalchemy import select, func
+
+from app.core.db import async_session_maker
+from app.models.transaction import Transaction
+from app.models.user import User
+
+
+def number_of_users() -> Callable[[Info], None]:
+    METRIC = Gauge(
+        "number_of_users_total",
+        "Number of registered users.",
+        labelnames=("users",)
+    )
+
+    async def instrumentation(info: Info) -> None:
+        try:
+            async with async_session_maker() as session:
+                result = await session.execute(select(func.count(User.id)))
+                user_count = result.scalar_one() or 0
+        except Exception:
+            # In case of DB errors, avoid crashing metrics endpoint
+            user_count = 0
+        METRIC.labels(users="total").set(user_count)
+
+    return instrumentation
+
+
+def number_of_transactions() -> Callable[[Info], None]:
+    METRIC = Gauge(
+        "number_of_transactions_total",
+        "Number of transactions stored.",
+        labelnames=("transactions",)
+    )
+
+    async def instrumentation(info: Info) -> None:
+        try:
+            async with async_session_maker() as session:
+                result = await session.execute(select(func.count()).select_from(Transaction))
+                transaction_count = result.scalar_one() or 0
+        except Exception:
+            # In case of DB errors, avoid crashing metrics endpoint
+            transaction_count = 0
+        METRIC.labels(transactions="total").set(transaction_count)
+
+    return instrumentation