public Optional<EffectiveMeta> findLatestBefore(final long timeMs) {
        if (isEmpty()) {
            return Optional.empty();
        } else {
            return Optional.ofNullable(effectiveMetas.floor(EffectiveMeta.asFloorKey(timeMs)));
        }
    }
Find the latest {@link EffectiveMeta} in effectiveMetaSet that is before or equal to timeMs.
stroom-meta/stroom-meta-api/src/main/java/stroom/meta/api/EffectiveMetaSet.java