To open the Met’s 2017–18 season, powerhouse soprano Sondra Radvanovsky tackled one of opera’s most fearsome roles, the title druid priestess of Bellini’s Norma, who wrestles with love and betrayal before making the ultimate sacrifice on a funeral pyre. Mezzo-soprano Joyce DiDonato sang Norma’s friend and rival, Adalgisa, in Sir David McVicar’s evocative production.