Support for with statement for Client class

[nnaka2992]

Is your feature request related to a problem? Please describe.
Currently, the Client class does not support with statement. Supporting with statement would make programmers easy to write and maintenance their code.

Describe the solution you'd like
Add __enter__ and __exit__ methods to enable with statement. Works as following would be perfect.

with bigquery.Client(location='Asia/Tokyo') as client:
    query_job = client.query("SELECT CURRENT_DATE('Asia/Tokyo');")

[plamut]

@nnaka2992 What would be the expected behavior here?

Presumably, __enter__() would simply return the client instance, i.e. self, and __exit__() would invoke the client's close() method?

This could be added, yes, thanks for the suggestion.

[nnaka2992]

Yes, that is what i meant. just return self and close is good enough.
thanks

[plamut]

OK great, this is exactly what I did in the PR, thanks for confirming!