Metallica made a deli dude's day when they rolled by his counter blaring their classic, "Enter Sandman" ... and he didn't hesitate to take lead vocals.

All of the metal legends -- James Hetfield, Lars Ulrich, Kirk Hammett and Robert Trujillo -- were in a Hollywood Gelson's supermarket Thursday with Billy Eichner from "Billy on the Street" ... and we were lucky enough to happen upon the live action. Heads were banging for sure, mostly behind the deli counter.