code: func TunnelSyncHealthChecker(tunneler Tunneler) func(req *http.Request) error {
  	return func(req *http.Request) error {
  		if tunneler == nil {
  			return nil
  		}
  		lag := tunneler.SecondsSinceSync()
  		if lag > 600 {
  			return fmt.Errorf("Tunnel sync is taking too long: %d", lag)
  		}
  		sshKeyLag := tunneler.SecondsSinceSSHKeySync()
  		// Since we are syncing ssh-keys every 5 minutes, the allowed
  		// lag since last sync should be more than 2x higher than that
  		// to allow for single failure, which can always happen.
  		// For now set it to 3x, which is 15 minutes.
  		// For more details see: http://pr.k8s.io/59347
  		if sshKeyLag > 900 {
  			return fmt.Errorf("SSHKey sync is taking too long: %d", sshKeyLag)
  		}
  		return nil
  	}
  }
docs: // TunnelSyncHealthChecker returns a health func that indicates if a tunneler is healthy.
  // It's compatible with healthz.NamedCheck