Basically title. Metallic dragons are covered in various types of metallic scales. Can a wizard target the scales to deal ongoing damage to the dragon?
Heat metal targets a manufactured metal object.
Heat metal (PHB, pg. 250) says (emphasis mine):
Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range.
A dragon's scales are not manufactured, rather are entirely natural, so are not eligible to be targeted by heat metal.
Metallic dragons have scales that appear to be metal but are not actually metal. The scales are organic and as such the spell cannot target the dragon. Unless this dragon is actually a construct in which case it can but some DM's might limit the effects of the spell since the construct does not process pain from heat the same way a creature would.