The Metropolitan Opera holds a celebration of Mirella Freni’s 40 years singing there. “At 70, Ms. Freni was reminding audiences that she would be by far the most interesting singer at her own gala. Not that the Met wasn’t offering some of its best to join her onstage; the program inadvertently seemed to confess that today’s stars are fewer and lesser than those who surrounded the honoree when she was in her prime a generation ago.”