code: private double calculateAverageExceptionRate(List<InvocationStat> invocationStats, long leastWindowCount) {
          long sumException = 0;
          long sumCall = 0;
          for (InvocationStat invocationStat : invocationStats) {
  
              long invocationLeastWindowCount = getInvocationLeastWindowCount(invocationStat,
                  ProviderInfoWeightManager.getWeight(invocationStat.getDimension().getProviderInfo()),
                  leastWindowCount);
  
              if (invocationLeastWindowCount != -1
                  && invocationStat.getInvokeCount() >= invocationLeastWindowCount) {
                  sumException += invocationStat.getExceptionCount();
                  sumCall += invocationStat.getInvokeCount();
              }
          }
          if (sumCall == 0) {
              return -1;
          }
          return CalculateUtils.divide(sumException, sumCall);
      }
docs: 计算平均异常率，如果调用次数小于leastWindowCount则不参与计算。 如果所有调用次数均为0则返回-1
  
  @param invocationStats List<InvocationStat>
  @param leastWindowCount leastWindowCount
  @return The average exception rate of all invocation statics