Bono wowed a crowd at Bemelmans Bar when the U2 rocker spontaneously sang Cole Porter. During Jim Caruso and Billy Stritch’s regular Sunday night gig at the Carlyle, spies saw Bono enter the bar with three pals. “Jim and Billy were playing standards from the Great American Songbook,” said a spy, “and Bono quietly grooved and sang along from his table. He knew just about every word to every song.” Sources say Bono and company stayed for two sets, and when they began to leave, the rocker headed toward the piano rather than the door. “He conferred quickly with Billy, then performed ‘I’ve Got You Under My Skin,’ ” our spy said. (That’s the same tune Bono sang with Frank Sinatra on Ol’ Blue Eyes’ 1993 album “Duets.”) “The crowd was stunned,” said a witness, adding, “As Bono finished the song and headed for the door, Billy said into the mic, ‘Well . . . that happened.’ ” One of Bono’s buds was overheard telling him he should play the Carlyle more often. Bono quipped, “Yeah, the Carlyle would be a nice change from the Garden,” and took off into the night.