private void renderSimpleField(XMLWriter writer, String relevancyFieldName, Relevance relevance) {
        writeOpenFieldElement(writer, relevancyFieldName);
        writer.content(relevance.toString(), false);
        writeCloseFieldElement(writer);
    }
Creates an XML 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/XmlRenderer.java