i have an allergy to nickel and chrome which are very commonly used. For me, the biggest problem is the rivets in my collars and cuffs. Wwe have covered these with moleskin which largely alleviates the problem. The buckle and D-ring still brush my skin a little but i paint them with clear nail polish which helps and my allergy is minimal and usually only becomes a problem after prolonged wear and is only a mild irritation even at that. i only wear my leather collar and cuffs during play. (my day collar is worn 24/7 but it is sterling silver expressly for this reason.) Wwe typically use rope instead of chain so that usually isn't an issue, however, chain attached to my cuffs or collar wouldn't be in contact with my skin much in the way Wwe would use it so this isn't much of a concern anyway.
Unless your bottom has a sensitivity though, this shouldn't be an issue. If you are concerned, you can simply spot test the metal on their skin for 20 minutes or so and see if they have a reaction while not playing or otherwise distracting and have benedryl on hand if you want to be super safe. Most metal allergies are not severe and usually take some time to become bothersome so it's highly unlikely for someone to have a previously undiscovered allergy to a common metal that would cause them to go into anaphylactic shock immediately upon touching it or some similarly severe reaction. Some metal blends won't begin to irritate me until i've been in contact with it for several hours. And my reaction goes away almost immediately upon removing the metal and washing with soap and water.