Up-and-coming vocalist LP treated guests at Brooklyn’s Bar Bruno to an impromptu concert Wednesday. After taping a performance for the “Late Show with David Letterman,” she and her band ventured to the Carroll Gardens eatery for dinner. During their meal, the broadcast of “Letterman” came on a TV, and LP began to sing along to her own performance and serenade diners with her hit “Into the Wild.” The band members jumped in to support her, entertaining customers with the spontaneous singalong.