Nicole Kidman and her husband, Keith Urban, were among the first celebrities to arrive inside the Great Hall at the Metropolitan Museum of Art on Monday for the Costume Institute benefit, honoring the designer Karl Lagerfeld.The couple was accompanied by Baz Luhrmann and his wife, Catherine Martin.For the cameras, Ms. Kidman, with her long, strawberry blond locks, in a swath of peachy fabric, seemed to float toward Mr. Luhrmann's side, in front of a towering centerpiece featuring hundreds of one-liter plastic bottles sourced from a recycling plant.