Time was, a Met concert performance of Rigoletto or Bohème could attract 35,000 people to Central Park. But this week’s three-singer-and-piano recital there only drew about 4,300 listeners, even with Tony-winning heartthrob Paulo Szot as the headliner. Will next month’s Summer HD Festival – outdoor re-screenings of past live hi-def broadcasts – be the future of the Met’s summer programming?