private void captureAggregateMetrics(final AggregateState aggregateState) {
        try {
            aggregateItemCountHistogram.update(aggregateState.itemCount);
            aggregateByteSizeHistogram.update(aggregateState.totalBytes);
            aggregateAgeHistogram.update(aggregateState.getAge().toMillis());
        } catch (Exception e) {
            LOGGER.error("Error capturing aggregate stats: {}", LogUtil.exceptionMessage(e), e);
        }
    }
Just get a single part for the entire file group.

@param fileGroup The file group to get the zip item count and total uncompressed size from.
@return A single part to add to teh current aggregate.
@throws IOException Could be throws when reading entries.
stroom-proxy/stroom-proxy-app/src/main/java/stroom/proxy/app/handler/PreAggregator.java