Every pitcher’s worst nightmare came true for Brandon Sproat. The former Mets prospect stepped up to the mound for the Milwaukee Brewers’ series finale on Sunday against the Chicago White Sox, allowing back-to-back walks and a single, leading to a grand slam by Colson Montgomery four batters into the first inning. Sproat, 25, did recover to get out of the inning, but ultimately ended the game in unspectacular fashion. The former high-profile prospect completed three innings with seven earned