docs: @BetaApi
    public final RegionInstanceGroupManagersListInstancesResponse
        listManagedInstancesRegionInstanceGroupManagers(String instanceGroupManager) {
  
      ListManagedInstancesRegionInstanceGroupManagersHttpRequest request =
          ListManagedInstancesRegionInstanceGroupManagersHttpRequest.newBuilder()
              .setInstanceGroupManager(instanceGroupManager)
              .build();
      return listManagedInstancesRegionInstanceGroupManagers(request);
    }