public static Long getLongHeader(Map<String, ?> args, String header, boolean required) throws RestServiceException {
    return getNumericalHeader(args, header, required, Long::parseLong);
  }
Gets the value of a header as a {@link Long}
@param args a map of arguments to be used to look for {@code header}.
@param header the name of the header.
@param required if {@code true}, {@link RestServiceException} will be thrown if {@code header} is not present
                in {@code args}.
@return the value of {@code header} in {@code args} if it exists. If it does not exist and {@code required} is
         {@code false}, then returns null.
@throws RestServiceException same as cases of {@link #getHeader(Map, String, boolean)} and if the value cannot be
                             converted to a {@link Long}.
ambry-api/src/main/java/com/github/ambry/rest/RestUtils.java