sdk: Flush metrics on exit (#749)

In PushController before exit, flush the meter by calling tick(), ensuring that all metrics are flushed.

Co-authored-by: alrex <aboten@lightstep.com>

opentelemetry-sdk/src/opentelemetry/sdk/metrics/export/controller.py

@@ -44,6 +44,8 @@ def run(self):
 
     def shutdown(self):
         self.finished.set()
+        # Run one more collection pass to flush metrics batched in the meter
+        self.tick()
         self.exporter.shutdown()
         if self._atexit_handler is not None:
             atexit.unregister(self._atexit_handler)

opentelemetry-sdk/tests/metrics/export/test_export.py

@@ -597,10 +597,15 @@ def test_push_controller(self):
         controller = PushController(meter, exporter, 5.0)
         meter.collect.assert_not_called()
         exporter.export.assert_not_called()
+
         controller.shutdown()
         self.assertTrue(controller.finished.isSet())
         exporter.shutdown.assert_any_call()
 
+        # shutdown should flush the meter
+        self.assertEqual(meter.collect.call_count, 1)
+        self.assertEqual(exporter.export.call_count, 1)
+
     def test_push_controller_suppress_instrumentation(self):
         meter = mock.Mock()
         exporter = mock.Mock()