WASHINGTON — Sen. Kirsten Gillibrand, D-N.Y., a central figure in the Democratic Party’s debate about the MeToo movement, is preparing to join a 2020 presidential primary contest that features a record number of women in the wake of the midterm election tidal wave of female candidates.Gillibrand, 52, is expected to announce formation of a presidential exploratory committee on “The Late Show With Stephen Colbert” on Tuesday night, in advance of a weekend trip to Iowa, where the first 2020 Democratic contest will be held.She will join a rapidly growing field that for the first time will offer Democrats a wide choice among female candidates.