ARLINGTON — Marcus Semien’s style of leadership for the first-place Rangers hasn’t been a secret throughout the league. He is not a “rah-rah” type of guy, but a strong, silent, lead-by-example type who mentors younger players and remains a steady presence at the top of the lineup and at second base, providing both Gold Glove-caliber defense and Silver Slugger-caliber offense. There’s a number in baseball — Wins Above Replacement — that legitimately quantifies a player’s importance to his club. And though Semien leads the Rangers in bWAR at 3.0, he has still been so much more to Texas. “It’s so…