Short answer: a should be used because copper is one one example under the category of useful metals which also could include aluminum, iron, gold, silver, among others.
In isolation, that is the answer most speakers would choose
Long Answer: It depends on the circumstance. To give one example:
An engineer is talking to their apprentice through why copper is (surprisingly to the apprentice) the metal used for a specific application
You would think steel would be used here, right? Most people do but this situation is different. Copper is the useful metal
This is a scenario where what is considered "useful" has been more narrowly defined, to where copper is the only member of the group (or at least the most useful of the members.) Therefore it is the metal as opposed to a metal.