Clojurians Log v2

Hey folks, I'm trying to understand how to add a custom client middleware for a request (as documented under https://github.com/ztellman/aleph/blob/98e878946a97fd020a63cc43ee6e740c712ad188/src/aleph/http.clj#L234) and I don't see how I'm supposed to use it? Obviously, :middleware should be a single-arity function, but what does it receive?