code: func (g *Cloud) ListInstancesInInstanceGroup(name string, zone string, state string) ([]*compute.InstanceWithNamedPorts, error) {
  	ctx, cancel := cloud.ContextWithCallTimeout()
  	defer cancel()
  
  	mc := newInstanceGroupMetricContext("list_instances", zone)
  	req := &compute.InstanceGroupsListInstancesRequest{InstanceState: state}
  	v, err := g.c.InstanceGroups().ListInstances(ctx, meta.ZonalKey(name, zone), req, filter.None)
  	return v, mc.Observe(err)
  }
docs: // ListInstancesInInstanceGroup lists all the instances in a given
  // instance group and state.