code: @Override
      public MeasureModel buildMeasureModel(InvocationStat invocationStat) {
          InvocationStatDimension statDimension = invocationStat.getDimension();
          String key = statDimension.getDimensionKey();
          MeasureModel measureModel = appServiceMeasureModels.get(key);
          if (measureModel == null) {
              measureModel = new MeasureModel(statDimension.getAppName(), statDimension.getService());
              MeasureModel oldMeasureModel = appServiceMeasureModels.putIfAbsent(key, measureModel);
              if (oldMeasureModel == null) {
                  measureModel.addInvocationStat(invocationStat);
                  return measureModel;
              } else {
                  oldMeasureModel.addInvocationStat(invocationStat);
                  return null;
              }
          } else {
              measureModel.addInvocationStat(invocationStat);
              return null;
          }
      }
docs: 如果该Invocation不属于一个MeasureModel，那么创建一个MeasureModel。并返回该MeasureModel。
  如果该Invocation属于一个MeasureModel，那么将该Invocation加入到该MeasureModel中。返回null。
  
  @param invocationStat InvocationStat
  @return MeasureModel