See googleapis/google-cloud-node#1542

We need to modify the headers being sent to:

headers = [
    ('authorization', 'Bearer ' + access_token),
    ('user-agent', self._user_agent),
    ('google-cloud-resource-prefix', 'projects/my-project-id'),
]

Though we may want to do this on a per-request basis. Probably not, since self._user_agent doesn't change after the MetadataPlugin object is created, even if the user agent on the original Client has changed.