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.

It all depends upon the process the printer uses. The best thing to do is to check with the entity who is actually going to be printing on the metal.

What should usually work with most providers of metal prints:

Rather than using JPEG, use a PNG with a transparent background or base. Place the parts of an image you want to be printed in a layer on top of the transparent base layer. You can do this with photo manipulation applications such as Photoshop or Gimp by using the selection tools to create a mask and only copy the parts you want to the layer of your PNG.