code: public static Collector<MonetaryAmount,GroupMonetarySummaryStatistics,GroupMonetarySummaryStatistics>
      groupBySummarizingMonetary(){
          return Collector.of(GroupMonetarySummaryStatistics::new, GroupMonetarySummaryStatistics::accept,
                              GroupMonetarySummaryStatistics::combine);
      }
docs: of MonetaryAmount group by MonetarySummary
  @return the MonetarySummaryStatistics