Initialize credentials after IO engine set

src/python/grpcio/grpc/_cython/_cygrpc/aio/grpc_aio.pyx.pxi

@@ -114,7 +114,7 @@ cdef _actual_aio_shutdown():
         raise ValueError('Unsupported engine type [%s]' % _global_aio_state.engine)
 
 
-cdef init_grpc_aio():
+cpdef init_grpc_aio():
     """Initialis the gRPC AsyncIO module.
     
     Expected to be invoked on critical class constructors.
@@ -126,7 +126,7 @@ cdef init_grpc_aio():
             _actual_aio_initialization()
 
 
-cdef shutdown_grpc_aio():
+cpdef shutdown_grpc_aio():
     """Shuts down the gRPC AsyncIO module.
 
     Expected to be invoked on critical class destructors.

src/python/grpcio/grpc/experimental/aio/__init__.py

@@ -20,7 +20,8 @@ created. AsyncIO doesn't provide thread safety for most of its APIs.
 from typing import Any, Optional, Sequence, Tuple
 
 import grpc
-from grpc._cython.cygrpc import (EOF, AbortError, BaseError, InternalError,
+from grpc._cython.cygrpc import (init_grpc_aio, shutdown_grpc_aio, EOF,
+                                 AbortError, BaseError, InternalError,
                                  UsageError)
 
 from ._base_call import (Call, RpcContext, StreamStreamCall, StreamUnaryCall,
@@ -39,6 +40,8 @@ from ._channel import insecure_channel, secure_channel
 ###################################  __all__  #################################
 
 __all__ = (
+    'init_grpc_aio',
+    'shutdown_grpc_aio',
     'AioRpcError',
     'RpcContext',
     'Call',

src/python/grpcio_tests/tests_aio/unit/_test_base.py

@@ -64,3 +64,6 @@ class AioTestBase(unittest.TestCase):
                 return _async_to_sync_decorator(attr, self._TEST_LOOP)
         # For other attributes, let them pass.
         return attr
+
+
+aio.init_grpc_aio()