code: public void addAdditionalField(String keyValue) {
          String[] splitted = keyValue.split(":");
  
          if (splitted.length != 2) {
  
              throw new IllegalArgumentException("additionalField must be of the format key:value, where key is the MDC "
                      + "key, and value is the GELF field name. But found '" + keyValue + "' instead.");
          }
  
          additionalFields.put(splitted[0], splitted[1]);
      }
docs: Add an additional field. This is mainly here for compatibility with logback.xml
  
  @param keyValue This must be in format key:value where key is the MDC key, and value is the GELF field
  name. e.g "ipAddress:_ip_address"