Pubsub: add 'PublisherClient.from_service_account_dict'

[xrmx]

Is your feature request related to a problem? Please describe.
I'd like to use the pubsub client on a webapp where i have the secrets in environment variables and not on disk.
Describe the solution you'd like
I would like to have a classmethod added to Publisher to instantiate a client with the credentials passed from a dict instead of a json file path.

[tseaver]

@xrmx You should be able to create credentials via the from_service_account_info method, and pass them to the client constructor, e.g.:

from google.oauth2.service_account import Credentials
from google.cloud.pubsub_v1 import PublisherClient

credentials = Credentials.from_service_account_info(your_dict)
publisher_client = PublisherClient(credentials=credentials)

[xrmx]

@tseaver sure I am currently doing this, but it would be nice to have already a wrapper to avoid digging into source code to find out where all this come from :)

[sduskis]

I think that this is a general auth problem. There should be a simple way to configure auth. IMHO, if there's a nee for a different auth configuration, there should be a clear and documented way to do more complex configuration. The changes should probably adding more docs for auth, and then getting references to those docs from all of the clients.

[tseaver]

The PublisherClient API docs should definitely document the args to PublisherClient.__init__ more clearly, including passing a pre-created credentials. Ditto for the SubscriberClient API docs.

Given that we fix the docs, I'd say we don't need to add a from_service_account_dict method to either client.

[plamut]

FWIW, there is an open PR that adds a quick example on how to use an other-than-default Credentials object. If that is deemed sufficient, we could close this issue.

Or, if "almost sufficient", the PR can be updated to cover this issue, too.

[xrmx]

@plamut commented on the PR, thanks!