How to include transparent parts in a print on metal

Whether single-colour [black & white] or 4 or more colour, anything white or even very light-coloured on screen or paper will be ‘metal’ once printed.

It would be unusual for a photo-print process to use a white ink without specifying; but check with your supplier to be certain.
Most print processes tend to rely on the material being printed on to provide the ‘white’ in any image. The only common time this isn’t always true is for such as t-shirt/fabric printing, where the underlying colour is not expected to be always white.