private void writeOpenFieldElement(XMLWriter writer, String relevancyFieldName) {
        Utf8String utf8 = fieldNameMap.get(relevancyFieldName);
        if (utf8 == null) {
            utf8 = new Utf8String(relevancyFieldName);
            fieldNameMap.put(relevancyFieldName, utf8);
        }
        writer.openTag(FIELD).attribute(NAME, utf8);
        writer.closeStartTag();
    }
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