def _refresh(self, http_request):
        response, content = http_request(
            META, headers={'Metadata-Flavor': 'Google'})
        content = _from_bytes(content)
        if response.status == http_client.OK:
            try:
                token_content = json.loads(content)
            except Exception as e:
                raise HttpAccessTokenRefreshError(str(e),
                                                  status=response.status)
            self.access_token = token_content['access_token']
        else:
            if response.status == http_client.NOT_FOUND:
                content += (' This can occur if a VM was created'
                            ' with no service account or scopes.')
            raise HttpAccessTokenRefreshError(content, status=response.status)
Refreshes the access_token.
dataset/ETHPy150Open google/oauth2client/oauth2client/contrib/gce.py/AppAssertionCredentials._refresh