Rob Thomas is the Zelig of the music industry. Over the past 30 years, the three-time Grammy winner fronted Matchbox 20, collaborated with Carlos Santana on “Smooth,” had a solo career all of his own and re-entered the zeitgeist when Greta Gerwig wrote his 1997 song “Push” into “Barbie.” “It’s funny,” Thomas, 53, told The Post in an exclusive interview. “When you’re younger and think about having a long career, you think about success in short, little terms. You don’t think about the long haul.