code: func (g *Cloud) AddInstancesToInstanceGroup(name string, zone string, instanceRefs []*compute.InstanceReference) error {
  	ctx, cancel := cloud.ContextWithCallTimeout()
  	defer cancel()
  
  	mc := newInstanceGroupMetricContext("add_instances", zone)
  	// TODO: should cull operation above this layer.
  	if len(instanceRefs) == 0 {
  		return nil
  	}
  	req := &compute.InstanceGroupsAddInstancesRequest{
  		Instances: instanceRefs,
  	}
  	return mc.Observe(g.c.InstanceGroups().AddInstances(ctx, meta.ZonalKey(name, zone), req))
  }
docs: // AddInstancesToInstanceGroup adds the given instances to the given
  // instance group.