AutoML: pass credentials from Table client to underlying client.

[shunghsiyu]

Problem

It seems that TablesClient doesn't correctly pass the credentials down to underlying clients.

Since I have neither a default credential nor GOOGLE_APPLICATION_CREDENTIALS set on my machine, I end up getting DefaultCredentialsError.

Environment details

os: macOS 10.14.2
python version: 3.7
google-cloud-automl==0.7.0

Code example

from google.cloud import automl_v1beta1
from google.oauth2 import service_account


credentials = service_account.Credentials.from_service_account_file('sa.json')
client = automl_v1beta1.TablesClient(
    credentials=credentials,
    project=credentials.project_id,
    region='us-central1',
)

Stacktrace

/Users/account/project/.venv/bin/python /Users/account/project/prediction_starter/main.py
Traceback (most recent call last):
  File "/Users/account/project/prediction_starter/main.py", line 9, in <module>
    region='us-central1',
  File "/Users/account/project/.venv/lib/python3.7/site-packages/google/cloud/automl_v1beta1/tables/tables_client.py", line 110, in __init__
    client_info=client_info_, **kwargs
  File "/Users/account/project/.venv/lib/python3.7/site-packages/google/cloud/automl_v1beta1/gapic/auto_ml_client.py", line 265, in __init__
    address=api_endpoint, channel=channel, credentials=credentials
  File "/Users/account/project/.venv/lib/python3.7/site-packages/google/cloud/automl_v1beta1/gapic/transports/auto_ml_grpc_transport.py", line 67, in __init__
    "grpc.max_receive_message_length": -1,
  File "/Users/account/project/.venv/lib/python3.7/site-packages/google/cloud/automl_v1beta1/gapic/transports/auto_ml_grpc_transport.py", line 104, in create_channel
    address, credentials=credentials, scopes=cls._OAUTH_SCOPES, **kwargs
  File "/Users/account/project/.venv/lib/python3.7/site-packages/google/api_core/grpc_helpers.py", line 177, in create_channel
    credentials, _ = google.auth.default(scopes=scopes)
  File "/Users/account/project/.venv/lib/python3.7/site-packages/google/auth/_default.py", line 317, in default
    raise exceptions.DefaultCredentialsError(_HELP_MESSAGE)
google.auth.exceptions.DefaultCredentialsError: Could not automatically determine credentials. Please set GOOGLE_APPLICATION_CREDENTIALS or explicitly create credentials and re-run the application. For more information, please see https://cloud.google.com/docs/authentication/getting-started

Expected Behavior

Successfully instantiate a TablesClient instance.