code: public String appMapString() {
          if (hasGroup()) {
              String appMap = Optional.ofNullable(this.fields.get(TracingConstants.APP_MAP)).orElse("");
              log.debug("App map: {}", appMap);
              return appMap;
          }
          raiseNonGroupException();
          return "{}";
      }
docs: String Type App map.
  
  @return appMap