There comes a time in the career of a famous rock band when its members are inexorably drawn to the idea of recording some homage to their influences, or perhaps a tribute to favored colleagues. For Metallica, this was the time, even when the hard-rocking quartet could get away with coasting. The ringer is an overly reverent, all-star version of Lynyrd Skynyrd’s lament, “Tuesday’s Gone,” recorded for a radio broadcast. But for the rest of these two discs, they take on a panoply of material from artists that range in diversity from lumpen, prole-metal band Merciful Fate to metal-pop cult faves the Misrts to aggro-punkers Discharge to blue-collar rocker Bob Seger, and Nick Cave, as well as Killing Joke! If it’s hell-bent guitar you want, don’t fret. Leave it to guitar legend Hammett, who stretches out on a monumental version of “Astronomy” from the songbook of riff-rock monsters Blue Oyster Cult. Metallica seem to have approached Garage Inc. with a spontaneity that cuts to the bone of the material. If it just happens to have an effortless feel to it, that’s part of its allure.