code: public GelfMessage addFields(Map<String, String> fields) {
  
          if (fields == null) {
              throw new IllegalArgumentException("fields is null");
          }
  
          getAdditonalFields().putAll(fields);
          return this;
      }
docs: Add multiple fields (key/value pairs)
  
  @param fields map of fields
  @return the current GelfMessage.