protected void renderHitGroupHead(HitGroup hitGroup) throws IOException {
        generator.writeStartObject();

        renderHitContents(hitGroup);
        if (getRecursionLevel() == 1)
            renderCoverage();

        ErrorHit errorHit = hitGroup.getErrorHit();
        if (errorHit != null)
            renderErrors(errorHit.errors());

        // the framework will invoke begin methods as needed from here
    }
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