Post Malone led the crowd in an epic sing-a-long salute to Toby Keith during his performance after Arlington, Texas’ American Rodeo on Saturday night (March 9.) Post — a genre-bending global hip hop star who’s gone country quite a bit over the past few years — needed only an acoustic guitar and a classic Keith song to have the crowd eating out of the palm of his hand. He sang “As Good as I Once Was,” Keith’s rip-roaring 2005 hit about balancing middle age against still having a little ill-advised youthful fun. Before the song’s last chorus, Post took a dramatic pause, then raised up his red So…