Fix #2248

src/python/src/grpc/_adapter/_intermediary_low.py

@@ -144,10 +144,11 @@ class Call(object):
     self._metadata.append((key, value))
 
   def premetadata(self):
-    return self._internal.start_batch([
+    result = self._internal.start_batch([
           _types.OpArgs.send_initial_metadata(self._metadata)
       ], _IGNORE_ME_TAG)
     self._metadata = []
+    return result
 
   def read(self, tag):
     return self._internal.start_batch([

src/python/src/grpc/_adapter/_intermediary_low_test.py

@@ -282,6 +282,9 @@ class EchoTest(unittest.TestCase):
     self.assertIn(server_trailing_binary_metadata_key, metadata)
     self.assertEqual(server_trailing_binary_metadata_value,
                      metadata[server_trailing_binary_metadata_key])
+    self.assertSetEqual(set(key for key, _ in finish_accepted.metadata),
+                        set((server_trailing_metadata_key,
+                             server_trailing_binary_metadata_key,)))
 
     server_timeout_none_event = self.server_completion_queue.get(0)
     self.assertIsNone(server_timeout_none_event)