Fix OpenTelemetry Prometheus metrics server startup

- Add _start_prometheus_server method to start HTTP server
- Register PrometheusMetricReader collector with prometheus_client REGISTRY
- Parse endpoint to extract host and port for HTTP server
- This fixes the issue where metrics endpoint was not accessible

ivatar/opentelemetry_config.py

@@ -129,6 +129,9 @@ class OpenTelemetryConfig:
             )
             metrics.set_meter_provider(meter_provider)
 
+            # Start Prometheus HTTP server for metrics endpoint
+            self._start_prometheus_server(prometheus_reader, prometheus_endpoint)
+
             logger.info(
                 f"OpenTelemetry metrics configured with Prometheus endpoint: {prometheus_endpoint}"
             )
@@ -137,6 +140,33 @@ class OpenTelemetryConfig:
             logger.error(f"Failed to setup OpenTelemetry metrics: {e}")
             self.enabled = False
 
+    def _start_prometheus_server(
+        self, prometheus_reader: PrometheusMetricReader, endpoint: str
+    ) -> None:
+        """Start Prometheus HTTP server for metrics endpoint."""
+        try:
+            from prometheus_client import start_http_server, REGISTRY
+
+            # Parse endpoint to get host and port
+            if ":" in endpoint:
+                host, port = endpoint.split(":", 1)
+                port = int(port)
+            else:
+                host = "0.0.0.0"
+                port = int(endpoint)
+
+            # Register the PrometheusMetricReader collector with prometheus_client
+            REGISTRY.register(prometheus_reader._collector)
+
+            # Start HTTP server
+            start_http_server(port, addr=host)
+
+            logger.info(f"Prometheus metrics server started on {host}:{port}")
+
+        except Exception as e:
+            logger.error(f"Failed to start Prometheus metrics server: {e}")
+            self.enabled = False
+
     def setup_instrumentation(self) -> None:
         """Set up OpenTelemetry instrumentation for various libraries."""
         try: