Black magic roofing caulk.Works great you can even put it on when it's wet.Will stop as soon as you put it on and it sticks to anything.Regular caulk won't work as good on a roof due to sun and moisture.Or stick as well to metal.
Some leaks in metal are caused by the nails backing out a bit. If you can wiggle the suspect nail with some pliers, go ahead and pull it out and replace it with a slightly bigger nail or better yet a screw. Be careful not to dent the roofing, as that would most likely accelerate leaking. Then dob on your sealant of choice (read the tubes and look for something rated for metal roofing). If the problem gets worse, it may be time to consider a new roof.