Yannick Nezet-Seguin, the Metropolitan Opera's music director, dressed in a blazing sapphire jacket and trim black pants, stood before a mirror backstage on a recent afternoon and smiled.Oh my God, it's so good, he said, waving his baton.I love it so much.There were three days until the opening of Puccini's La Boheme, and Nezet-Seguin, surrounded by a small team of tailors, designers and assistants, was offering feedback on his attire, which had been designed by the Met's costume shop.