pottery shed / Converted Farm shed,
Adding poly will be adding a second vapor barrier. The kraft paper is the first and the only one you need.

This is not part of your question but I  would be concerned about possible condensation on the metal siding, and controlling that, if you are in a climate where that occurs. In Maryland I never seen that as a problem with my shop that had a metal roof, but now I am in WA state and the condensation is so heavy, it is like it is raining inside our metal barn. The metal siding has it as bad, but it just runs down to the gravel floor, the framing is galvanized metal so that will not trouble me. The dripping sure does though. Learned something there.