Peter Gelb got his first job at the Met, as an usher, when he was still a teenager. Now he’ll be expected to continue Joe Volpe’s successful run, and to steer the massive company through an increasingly difficult time for classical music in general. Gelb’s years at Sony Classical made him a target for purists – he shut down several specialty sub-labels and made “crossover” music a priority – and some longtime Met observers wonder if he will take a similarly commercial approach in his new post.