def __call__(self, unused_context, callback):
        access_token = self._credentials.get_access_token().access_token
        headers = [
            ('Authorization', 'Bearer ' + access_token),
            ('User-agent', self._user_agent),
        ]
        callback(headers, None)
Adds authorization header to request metadata.
dataset/ETHPy150Open GoogleCloudPlatform/gcloud-python/gcloud/bigtable/client.py/_MetadataPlugin.__call__