Live music is back, which means grainy cell-phone videos of strange live-music moments are also back. Last night, in a show that was announced just a couple of weeks ago, the Foo Fighters played at Madison Square Garden — the venue’s first indoor full-capacity live show in more than a year. We’ll have more on that show later, but the big news was the the night featured one superstar guest appearance. Near the end of the show, two of the world’s most famous Daves got together onstage.