code: func (id *IdentityID) HostID() (string, error) {
  	parts := strings.Split(id.HostUUID, ".")
  	if len(parts) < 2 {
  		return "", trace.BadParameter("expected 2 parts in %q", id.HostUUID)
  	}
  	return parts[0], nil
  }
docs: // HostID is host ID part of the host UUID that consists cluster name