迅速3中的map [string] interface {}

How can I implement map[string]interface{} in swagger 3? I have a struct field in golang named metadata with type map[string]interface{} because I want it free-form, no specific key. I have implemented it in swagger 3 like this:

metadata:
    type: 'object'
    description: 'Extra data of a collection'
    additionalProperties: true
    example: 
        layout: 'grid-3'
        max-feed-item: '20'

But I get this result:

The example is not shown. Why?