code: func (t *Tag) Proto() *api.Tag {
  	kv := strings.SplitN(t.Tag, ":", 2)
  	if len(kv) != 2 {
  		panic(fmt.Sprintf("bad tag %q", t.Tag))
  	}
  	return &api.Tag{
  		Key:        kv[0],
  		Value:      kv[1],
  		AttachedBy: t.RegisteredBy,
  		AttachedTs: google.NewTimestamp(t.RegisteredTs),
  	}
  }
docs: // Proto returns cipd.Tag proto with information from this entity.
  //
  // Assumes the tag is valid.