@Override
    public void beginList(DataList<?> list) throws IOException {
        moreChildren();
        if (list instanceof HitGroup) {
            renderHitGroupHead((HitGroup) list);
        } else if (list instanceof EventStream) {
            renderHitGroupHead(new HitGroup("event_stream"));  // Consider waiting for all events and create a single summary hit
        } else {
            throw new IllegalArgumentException("Expected subclass of com.yahoo.search.result.HitGroup, got " + list.getClass());
        }
    }
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