Ability to add other parameters like `debug` or `rows`

I want to add a parameter to return the count of documents (thinking maybe this is faster). This is exactly what is asked here:
https://stackoverflow.com/questions/5050746/solr-solrj-how-can-i-determine-the-total-number-of-documents-in-an-index

So in the answers we propose two solutions: debug or rows but for each, they provide a value like:

debug=query and rows=0. For what I've found in this library, is at least the solrClient#set but this one appears to take only one param, the key. I'm not sure if this would work.

Set a new parameter Since all possibilities provided by Solr are not available in the Query object, set() is there to fit this gap.

My question is how to set a key with a value ? since the signature does not allow that.

Furthermore now that I tested that, I'm having TypeError: query.set is not a function