code: public Observable<List<BackupManagementUsageInner>> listAsync(String vaultName, String resourceGroupName) {
          return listWithServiceResponseAsync(vaultName, resourceGroupName).map(new Func1<ServiceResponse<List<BackupManagementUsageInner>>, List<BackupManagementUsageInner>>() {
              @Override
              public List<BackupManagementUsageInner> call(ServiceResponse<List<BackupManagementUsageInner>> response) {
                  return response.body();
              }
          });
      }
docs: Fetches the backup management usage summaries of the vault.
  
  @param vaultName The name of the recovery services vault.
  @param resourceGroupName The name of the resource group where the recovery services vault is present.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the List&lt;BackupManagementUsageInner&gt; object