This string is used as the contents of a select tag's option. I am not sure using CSS here would work as intended - even if we defined a special class for "meta" entries, something like 
```
option.meta:before {content:"["}
option.meta:after {content:"]"}
```
it would display fine within the list, but the selected item would be displayed without the braces which does not match the current behavior. Maybe there's a CSS trick I don't know about - I'm by no means an expert in this area.

The way I did it here is consistent with other use cases elsewhere in filter api.