fix unit test coverage

Author: wangzlei

aws-opentelemetry-distro/tests/amazon/opentelemetry/distro/test_otlp_udp_exporter.py

@@ -92,6 +92,7 @@ def test_export_with_exception(self, mock_udp_exporter, mock_encode_metrics):
     def test_shutdown(self, mock_udp_exporter):
         mock_udp_exporter_instance = mock_udp_exporter.return_value
         exporter = OTLPUdpMetricExporter()
+        exporter.force_flush()
         exporter.shutdown()
         mock_udp_exporter_instance.shutdown.assert_called_once()
 
@@ -141,4 +142,5 @@ def test_shutdown(self, mock_udp_exporter):
         mock_udp_exporter_instance = mock_udp_exporter.return_value
         exporter = OTLPUdpSpanExporter()
         exporter.shutdown()
+        exporter.force_flush()
         mock_udp_exporter_instance.shutdown.assert_called_once()