Hidden in plain sight for decades, the megastar steps out in eye-popping style for the Met Gala, reveling in her turn as a new favorite of fashion. “I’m never going to take it off,” Celine Dion said. Mugging for a reporter’s iPhone camera with a voice you might associate with Dalmatian coats and Cruella De Vil, she repeated herself. “I’m never going to take it off,” she said, head lowered, tone rising. “It’s mine! All mine!” The subject wasn’t what she was wearing at the moment — a green metallic silk jumpsuit from Isabel Marant and Alexander Wang ankle boots — but an outfit by Oscar de la Renta, the one she will have on when the paparazzi go berserk on the Met Gala red carpet on Monday night. It is a creation that would seem as if designed specifically to test the internet’s breaking point.