code: @BetaApi
    public final InstanceGroupManagersListManagedInstancesResponse
        listManagedInstancesInstanceGroupManagers(
            ProjectZoneInstanceGroupManagerName instanceGroupManager) {
  
      ListManagedInstancesInstanceGroupManagersHttpRequest request =
          ListManagedInstancesInstanceGroupManagersHttpRequest.newBuilder()
              .setInstanceGroupManager(
                  instanceGroupManager == null ? null : instanceGroupManager.toString())
              .build();
      return listManagedInstancesInstanceGroupManagers(request);
    }
docs: Lists all of the instances in the managed instance group. Each instance in the list has a
  currentAction, which indicates the action that the managed instance group is performing on the
  instance. For example, if the group is still creating an instance, the currentAction is
  CREATING. If a previous action failed, the list displays the errors for that failed action.
  
  <p>Sample code:
  
  <pre><code>
  try (InstanceGroupManagerClient instanceGroupManagerClient = InstanceGroupManagerClient.create()) {
  ProjectZoneInstanceGroupManagerName instanceGroupManager = ProjectZoneInstanceGroupManagerName.of("[PROJECT]", "[ZONE]", "[INSTANCE_GROUP_MANAGER]");
  InstanceGroupManagersListManagedInstancesResponse response = instanceGroupManagerClient.listManagedInstancesInstanceGroupManagers(instanceGroupManager);
  }
  </code></pre>
  
  @param instanceGroupManager The name of the managed instance group.
  @throws com.google.api.gax.rpc.ApiException if the remote call fails