Yannick Nézet-Séguin, the next music director of the Metropolitan Opera, conducts a rehearsal of "Elektra" in New York, Feb. 23, 2018. A long day spent with Nézet-Séguin suggests that he is already changing the culture at an opera house facing shaky finances, a box-office slump and the fallout from his predecessor’s tumultuous final years. (Julieta Cervantes/The New York Times)