private void renderSpecialCasesForGrouping(Hit hit) throws IOException {
        if (hit instanceof AbstractList) {
            renderGroupingListSyntheticFields((AbstractList) hit);
        } else if (hit instanceof Group) {
            renderGroupingGroupSyntheticFields(hit);
        }
    }
Creates a json renderer using a custom executor.
Using a custom executor is useful for tests to avoid creating new threads for each renderer registry.
container-search/src/main/java/com/yahoo/search/rendering/JsonRenderer.java