Bu Sun Kim

Running `docgen` appears to result in the async credentials files getting deleted. Investigate and modify the session so files are not deleted.

Implement #665 #679 for the async credentials.

There is a limit of 210 AppEngine versions. Tests should ideally delete the version they've deployed so we don't need to manually clear versions. > ERROR: (gcloud.app.deploy) INVALID_ARGUMENT: Your app...

See https://github.com/googleapis/google-auth-library-python/pull/426#discussion_r368214783 for details

Use protoc-docs-plugin to add docs

1

Use protoc-docs-plugin to add docstrings to generated `_pb2` files. Add the docs related scripts and configs in `.kokoro`.