Accessing attributes from an AngularJS directive

My AngularJS template contains some custom HTML syntax like:

<su-label tooltip="{{field.su_documentation}}">{{field.su_name}}</su-label>

I created a directive to process it:

.directive('suLabel', function() {
  return {
    restrict: 'E',
    replace: true,
    transclude: true,
    scope: {
      title: '@tooltip'
    },
    template: '<label><a href="#" rel="tooltip" title="{{title}}" data-placement="right" ng-transclude></a></label>',
    link: function(scope, element, attrs) {
      if (attrs.tooltip) {
        element.addClass('tooltip-title');
      }
    },
  }
})

Everything works fine, at the exception of the attrs.tooltip expression, which always returns undefined, even though the tooltip attribute is visible from Google Chrome's JavaScript console when doing a console.log(attrs).

Any suggestion?

UPDATE: A solution was offered by Artem. It consisted in doing this:

link: function(scope, element, attrs) {
  attrs.$observe('tooltip', function(value) {
    if (value) {
      element.addClass('tooltip-title');
    }
  });
}

AngularJS + stackoverflow = bliss