Example lyrics given by the ESRB reveal two of the track pack's songs: "D.O.A." by Haunted and "Transmaniacon MC" by Blue Oyster Cult, a brand new addition to the Rock Band franchise. A leaked achievement list also reveals "Killed by Death" by Motorhead, "Hair of the Dog" by Nazareth, and "Master of the Universe" by Hawkwind as new songs exclusive to the pack. Considering the last Rock Band track pack debuted 13 new songs unavailable as DLC, we're confident that this new addition will do the same.
Rock Band: Metal Track Pack hasn't been announced yet, but considering the reliability of the ESRB's ratings, we expect EA and MTV Games to announce the full track list, along with the track pack itself, within the next few weeks.
[Thanks, Justin R.!]