Fans had been speculating about the appearance for weeks after Metallica scheduled a show at The Apollo Theater in Harlem on the same weekend as the ceremony honoring Rivera. "Enter Sandman" has become one of the last symbols of Rivera as it is the song played whenever he enters a game at Yankee Stadium.
Here is video of the ceremony (via L.A. Times)...
Here is the group presenting an autographed graphic to Rivera after the song...