_token_async
        import google.auth.transport.aiohttp_requests

        request = google.auth.transport.aiohttp_requests.Request()
        target_audience = "https://pubsub.googleapis.com"

        id_token = await google.oauth2._id_token_async.fetch_id_token(request, target_audience)

    Args:
        request (google.auth.transport.aiohttp_requests.Request): A callable used to make
            HTTP requests.
        audience (str): The audience that this ID token is intended for.

    Returns:
        str: The ID token.

    Raises:
        ~google.auth.exceptions.DefaultCredentialsError:
            If metadata server doesn't exist and no valid service account
            credentials are found.
    zCGOOGLE_APPLICATION_CREDENTIALS path is either not found or invalid.