code: public List<BillingStats> getBillingStats(GroupFilter groupFilter) {
          return findLazy(groupFilter)
              .map(
                  groupMetadata -> converter.convertBillingStats(
                      client.getGroupBillingStats(groupMetadata.getId())
                  )
              )
              .collect(toList());
      }
docs: Get billing stats by GroupFilter
  @param groupFilter group filter
  @return List of Group billing stats