code: public List<BillingStats> getBillingStats(Group... groups) {
          List<BillingStats> result = new ArrayList<>();
          List<Group> groupList = Arrays.asList(groups);
  
          groupList.forEach(
              group -> result.add(
                  converter.convertBillingStats(
                      client.getGroupBillingStats(idByRef(group))
                  )
              )
          );
  
          return result;
      }
docs: Get billing stats by Groups
  
  @param groups groups array
  @return List of Group billing stats