code: func (a *AuthServer) autoRotateCertAuthorities() error {
  	clusterName, err := a.GetClusterName()
  	if err != nil {
  		return trace.Wrap(err)
  	}
  	for _, caType := range []services.CertAuthType{services.HostCA, services.UserCA} {
  		ca, err := a.Trust.GetCertAuthority(services.CertAuthID{
  			Type:       caType,
  			DomainName: clusterName.GetClusterName(),
  		}, true)
  		if err != nil {
  			return trace.Wrap(err)
  		}
  		if err := a.autoRotate(ca); err != nil {
  			return trace.Wrap(err)
  		}
  	}
  	return nil
  }
docs: // autoRotateCertAuthorities automatically rotates cert authorities,
  // does nothing if no rotation parameters were set up
  // or it is too early to rotate per schedule