I see no-one has actually told you the answer LOL ! - it's no, Metformin nor any other drug used for diabetes (even insulin) will have any affect whatsoever on hard skin. It's a matter of hydration firstly and secondly, taking proper care of the tootsies.
A proper fitting shoe/boot is also key and just because it's a workboot doesn't mean it can't or won't, also be comfy. They do some belting steel toecap trainers these days too - things have moved on A LOT since our dads were needing to wear them! You may well have to pay more to get some that are - but these here feet have to last us our whole life - so they need to be treated as important!