Pfizer's plan to enter the booming obesity market took an expensive new turn.Shares of a little-known biotechnology stock called Metsera (MTSR 60.80%) popped on Monday, Sept. 22, after Pfizer (PFE 0.04%) announced it would be acquiring it for a steep premium. Under the terms of the agreement, Pfizer will spend $47.50 per share, or about$4.9 billion up front.In addition to an up-front payment that is 43% above the previous trading day's closing price, Metsera shareholders will receive a contingent value rig ...