Counting only citizens does preclude counting all free persons. (If you're speaking about counting all but considering citizenship a demographic, that's legal and used to be done iirc, but was dropped as it was feared it would disincentivize non-citizen being counted. The recent court case about the citizenship question was settled by the courts because of administrative procedure, not the legality of the question.)

Moreover, collecting demographics data is orthogonal and I'm not sure why you're bringing it up.