The world of tenors has expanded exponentially, it would seem, since the days when Luciano Pavoratti and Placido Domingo dominated the big tenor roles and the attention of the media and opera-loving public. Domingo, astonishingly, is still singing brilliantly, conducting, and running an opera company, but recently there have been a good half dozen excellent youngish tenors singing at the Met, including the fabulously gifted lyric tenor Matthew Polenzani.