I have a metal building already. It had no ridge vents or anything.
Before storing hay in there, I added some louvered vents down in the end of the building near the hay storage, and may need to add a powered fan (dustproof of course) to encourage circulation since it does get hot and muggy in there. A ridge vent could allow birds, critters, and rain in so I’m not necessarily suggesting one is necessary.
But our only evidence of mold came from the moist floor. I strongly recommend buying a bunch of pallets and 7/16" OSB plywood and making a raised deck to avoid stacking the hay on the floor. It helps if the pallets are the same length/width/height so that they make a nice level deck. OSB may be better than premium plywood, as the lack of veneer gives you better grip to walk on.