Get all occuring fields in query result

For a typical interactive product search application I would like to get all fields that are used in documents of a document set that has already been filtered.

I know how to formulate a query to retrieve the documents currently in focus.
I also found the GET MAPPING api to find details about all fields used in the index.

How to combine both information demands? Focusing on the documents retrieved by the filter query: What fields are in at least one of the documents (so it makes sense to use this field for further restricting of the result set).

Many thanks in advance,

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