Der Rosenkavalier is an opera about the passing of time and the need to let go. It’s a particularly appropriate, then, that the current Metropolitan Opera production marks goodbyes for two singers. Superstar soprano Renée Fleming is leaving the opera stage (though she will continue to perform in concerts and on Broadway), while mezzo-soprano Elina Garança has announced her intention to give up trouser roles (including, of course, Octavian). Elina will be very much...