code: @BetaApi
    public final RegionInstanceGroupManagersListInstancesResponse
        listManagedInstancesRegionInstanceGroupManagers(
            ProjectRegionInstanceGroupManagerName instanceGroupManager) {
  
      ListManagedInstancesRegionInstanceGroupManagersHttpRequest request =
          ListManagedInstancesRegionInstanceGroupManagersHttpRequest.newBuilder()
              .setInstanceGroupManager(
                  instanceGroupManager == null ? null : instanceGroupManager.toString())
              .build();
      return listManagedInstancesRegionInstanceGroupManagers(request);
    }
docs: Lists the instances in the managed instance group and instances that are scheduled to be
  created. The list includes any current actions that the group has scheduled for its instances.
  
  <p>Sample code:
  
  <pre><code>
  try (RegionInstanceGroupManagerClient regionInstanceGroupManagerClient = RegionInstanceGroupManagerClient.create()) {
  ProjectRegionInstanceGroupManagerName instanceGroupManager = ProjectRegionInstanceGroupManagerName.of("[PROJECT]", "[REGION]", "[INSTANCE_GROUP_MANAGER]");
  RegionInstanceGroupManagersListInstancesResponse response = regionInstanceGroupManagerClient.listManagedInstancesRegionInstanceGroupManagers(instanceGroupManager);
  }
  </code></pre>
  
  @param instanceGroupManager The name of the managed instance group.
  @throws com.google.api.gax.rpc.ApiException if the remote call fails