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