feat(api): complete Phase 2 - framework-less asyncio Ingestion API with backpressure and metrics

app/api/metrics.py

@@ -0,0 +1,37 @@
+import threading
+
+
+class APIMetrics:
+    """
+    In-memory metrics tracker for the Ingestion API.
+    Thread-safe and async-safe counter for Prometheus scraping.
+    """
+
+    def __init__(self):
+        self._lock = threading.Lock()
+        self._received_total: int = 0
+
+    def increment_received(self, count: int = 1) -> None:
+        with self._lock:
+            self._received_total += count
+
+    @property
+    def total_received(self) -> int:
+        with self._lock:
+            return self._received_total
+
+    def get_prometheus_metrics(self) -> str:
+        """
+        Formats metrics into standard Prometheus text format.
+        """
+        with self._lock:
+            val = self._received_total
+
+        return (
+            "# HELP cdr_api_received_total Total CDRs received by the API\n"
+            "# TYPE cdr_api_received_total counter\n"
+            f"cdr_api_received_total {val}\n"
+        )
+
+
+api_metrics = APIMetrics()