Possible to fetch only tags?

We would like to fetch a list of all tags used by a particular document type, as we were planning on using tags to show filters for the data. We were using the tags returned in the initial API response, but this seems to be being deprecated:
https://prismic.io/blog/a-change-to-how-the-prismic-api-handles-tags

The natural replacement for this would be to include all used tags in the query header. Sadly, my query to try and fetch just the tags seems to be returning all data from all documents:

$query = [];
$query[] = Predicates::at(
    'document.type',
    $documentType
);
$options = ['fetch'=> 'document.tags'];
$response = $api->query($query, $options);

Is there a proper way to fetch this info? Is there a way to exclude documents that have no tags?

Hi there,

Thanks for letting us know of your use-case

I have reported that to our production team and we will let you know once we have any new updates.

Best,
Fares