Peter Gelb has been sending every possible signal that the Metropolitan Opera is a new place under his leadership — the kind of place where great art intersects with mainstream American life. And now this: “Opera buffs can get a sneak peak at the Met’s upcoming production of Rossini’s ‘Il Barbiere di Siviglia’ (that’s ‘The Barber of Seville’ for all of you barbarians) tomorrow on, of all places, ‘Late Show With David Letterman.’ It will be the first time Letterman, an opera fan, has had an opera production as his musical guest.”