dd x-goog-api-client header with the given value.

    Args:
        headers (Mapping[str, str]): The headers to which we will add the
            metric header.
        metric_header_value (Optional[str]): If value is None, do nothing;
            if headers already has a x-goog-api-client header, append the value
            to the existing header; otherwise add a new x-goog-api-client
            header with the given value.
    Nú