If it goes
through the metal, you've got serious problems...
If you can just see metal, you have a few options. Either leave it be (your forks will rust, your frame should be fine), put a sticker on it like I do
or paint over it. Depending on how nice you like your bike to look, how skilled you are at painting, how much you're willing to spend and what materials you have available, you can do one of the following: use a spray can and just blow over the spot or the whole frame in whatever colour you choose (not very nice looking), get a professional to respray the whole thing (not cheap), or using either modeler's enamel paint and a brush or a touch-up brush from your local Halford's (UK) Schuck's (US) etc, carefully paint the scratch with the closest match you can find or mix and sand it smooth with 400 grit sandpaper (soak it in water first, you'll get it a lot smoother) and polish it with car wax.