Tan Dun’s “The First Emperor,” Placido Domingo’s first world premiere at the Metropolitan Opera, comes at a highly productive time in the legendary tenor’s career. “But Domingo, despite an almost unrivaled longevity, also knows that the end will come someday. … ‘Right now for me it’s a mystery of how long I’m going to sing,’ ” he said. ” ‘I have about three or four new operas I will still be doing. It’s exciting. But I’m 65 now; I’ll be very surprised if I will be singing opera when I’m 70. Perhaps I’ll do concerts, but I don’t think operas.’ “