Olivia Rodrigo rocked out on guitar during “Obsessed” in the first of her four “Guts” World Tour shows at MSG. She was accompanied by a troupe of interpretive dancers during “Vampire,” the piano ballad that became her most recent No. 1 single last July. With only two studio albums — “Sour” and “Enough for You” — she performed for an hour and 45 minutes without it feeling like there was too much filler