code: @BetaApi
    public final ListInstancesRegionInstanceGroupsPagedResponse listInstancesRegionInstanceGroups(
        String instanceGroup,
        RegionInstanceGroupsListInstancesRequest regionInstanceGroupsListInstancesRequestResource) {
      ListInstancesRegionInstanceGroupsHttpRequest request =
          ListInstancesRegionInstanceGroupsHttpRequest.newBuilder()
              .setInstanceGroup(instanceGroup)
              .setRegionInstanceGroupsListInstancesRequestResource(
                  regionInstanceGroupsListInstancesRequestResource)
              .build();
      return listInstancesRegionInstanceGroups(request);
    }
docs: Lists the instances in the specified instance group and displays information about the named
  ports. Depending on the specified options, this method can list all instances or only the
  instances that are running.
  
  <p>Sample code:
  
  <pre><code>
  try (RegionInstanceGroupClient regionInstanceGroupClient = RegionInstanceGroupClient.create()) {
  ProjectRegionInstanceGroupName instanceGroup = ProjectRegionInstanceGroupName.of("[PROJECT]", "[REGION]", "[INSTANCE_GROUP]");
  RegionInstanceGroupsListInstancesRequest regionInstanceGroupsListInstancesRequestResource = RegionInstanceGroupsListInstancesRequest.newBuilder().build();
  for (InstanceWithNamedPorts element : regionInstanceGroupClient.listInstancesRegionInstanceGroups(instanceGroup.toString(), regionInstanceGroupsListInstancesRequestResource).iterateAll()) {
  // doThingsWith(element);
  }
  }
  </code></pre>
  
  @param instanceGroup Name of the regional instance group for which we want to list the
  instances.
  @param regionInstanceGroupsListInstancesRequestResource
  @throws com.google.api.gax.rpc.ApiException if the remote call fails