feat: Add support for server-side flow control (#143)

* chore: Remove notes about ordering keys being experimental.

* Revert "chore: Remove notes about ordering keys being experimental."

This reverts commit 38b2a3e.

* feat: Add support for server-side flow control

* Add unit test for flow control

google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py

@@ -575,6 +575,8 @@ def _get_initial_request(self, stream_ack_deadline_seconds):
             stream_ack_deadline_seconds=stream_ack_deadline_seconds,
             subscription=self._subscription,
             client_id=self._client_id,
+            max_outstanding_messages=self._flow_control.max_messages,
+            max_outstanding_bytes=self._flow_control.max_bytes,
         )
 
         # Return the initial request.

tests/unit/pubsub_v1/subscriber/test_streaming_pull_manager.py

@@ -159,6 +159,15 @@ def test_client_id():
     assert client_id_1 != client_id_2
 
 
+def test_streaming_flow_control():
+    manager = make_manager(
+        flow_control=types.FlowControl(max_messages=10, max_bytes=1000)
+    )
+    request = manager._get_initial_request(stream_ack_deadline_seconds=10)
+    assert request.max_outstanding_messages == 10
+    assert request.max_outstanding_bytes == 1000
+
+
 def test_ack_deadline_with_max_duration_per_lease_extension():
     manager = make_manager()
     manager._flow_control = types.FlowControl(max_duration_per_lease_extension=5)