private void addItemCountMetric(Query query, Metric.Context context) {
        metric.set(QUERY_ITEM_COUNT, query.getModel().getQueryTree().treeSize(), context);
    }
Effectively flattens the hits, and measures relevance @ 1, 3, and 10
container-search/src/main/java/com/yahoo/prelude/statistics/StatisticsSearcher.java