@Override
    public void writePayload(final Output output) {
        throw new RuntimeException("Not implemented");
//        Metrics.measure("writePayload", () -> {
//            final List<byte[]> list = new ArrayList<>();
//            childMap.keySet().forEach(groupKey -> {
//                final ItemsImpl items = childMap.remove(groupKey);
//                if (items != null) {
//                    list.addAll(items.getList());
//                }
//            });
//
//            output.writeInt(list.size());
//            for (final byte[] item : list) {
//                output.writeInt(item.length);
//                output.writeBytes(item);
//            }
//        });
    }
Write data from the data store to an output removing them from the datastore as we go as they will be transferred
to another store.

@param output The output to write to.
stroom-query/stroom-query-common/src/main/java/stroom/query/common/v2/MapDataStore.java