@Override
    public Optional<RawFlag> fetch(FlagId id, FetchVector vector) {
        return sources.stream()
                .map(source -> source.fetch(id, vector))
                .filter(Optional::isPresent)
                .map(Optional::get)
                .findFirst();
    }
@param sources Flag sources in descending priority order.
flags/src/main/java/com/yahoo/vespa/flags/OrderedFlagSource.java