Renders the grouped tag values in the MultiSelect
(see example).
The template context is set to the current component.
To get a reference to the current grouped
data items collection, use the let-dataItems directive.

The GroupTagTemplate directive can only be used with the MultiSelect component.

selector: 'my-app',
template: `
 <kendo-multiselect kendoMultiSelectSummaryTag [data]="items">
   <ng-template kendoMultiSelectGroupTagTemplate let-dataItems>
     <span>{{dataItems.length}} item(s) selected</span>
class AppComponent {
  public items: Array<string> = ["Item 1", "Item 2", "Item 3", "Item 4"];