Add list command for AggregateCounter
  Added list REST endpoint at AggregateCountersController
  Made AggregateCount implement Metric
  Updated XDShell command API for list
Fix delete operation for AggregateCounterRepository
  Fixed deleting aggregatecounter's metric meta 'set' when deleting it.
Fix review comments
  Fixed AggregateCount not to implement Metric
  Added tests for aggregatecounter list, delete
  Added AbstractMetricIntegrationTest abstract class for metric helper methods
  Updated other shell tests with the changes
XD-619 Add list command for AggregateCounter