code: func logTokenError(c context.Context, r *http.Request, tok *tokenserver.MachineTokenBody, err error, msg string, args ...string) {
  	fields := logging.Fields{"remoteAddr": r.RemoteAddr}
  	if tok != nil {
  		// Note that if token wasn't properly signed, these fields may contain
  		// garbage.
  		fields["machineFqdn"] = tok.MachineFqdn
  		fields["issuedBy"] = tok.IssuedBy
  		fields["issuedAt"] = tok.IssuedAt
  		fields["lifetime"] = tok.Lifetime
  		fields["caId"] = tok.CaId
  		fields["certSn"] = tok.CertSn
  	}
  	if err != nil {
  		fields[logging.ErrorKey] = err
  	}
  	fields.Warningf(c, msg, args)
  }
docs: // logTokenError adds a warning-level log entry with details about the request.