@Override
    public String toString() {
        return "RefDataLookupRequest{" +
               "mapName='" + mapName + '\'' +
               ", key='" + key + '\'' +
               ", effectiveTimeEpochMs="
               + NullSafe.toStringOrElse(effectiveTimeEpochMs, Instant::ofEpochMilli, "null") +
               ", referenceLoaders=" + referenceLoaders +
               '}';
    }
@return data string or epoch ms
stroom-pipeline/src/main/java/stroom/pipeline/refdata/RefDataLookupRequest.java