Feed them directly to whatever you're wanting to level up, especially if they're sharing the same element (+50% EXP bonus), but this isn't necessary if your monster box is full or you're not raising anything of a certain metal dragon's elemental type... it's not a big deal if they don't match. Regular metal dragons give 1000 EXP (1500 if it's of the same elemental type), the next level gives 10,000 EXP, then 50,000, and then one that is ~82,500 I believe. Those are the only ones I've seen... definitely hit those metal dragon dungeons up.