“You can spend a decade singing minor roles in residence at a major opera house. And another 10 years in the hinterlands, perfecting major roles with minor opera companies. And by the time your ducks are lined up, you’re lucky to have a few years of good singing left, since opera stars, like professional athletes, have careers circumscribed by age and time. Too much too soon can kill a young voice. But too little too late can waste a singer’s prime.” So when a singer gets an opportunity to step in on short notice to sing at the Met…