* Foot traffic also rose 2.5 pct-ShopperTrak * Heavy discounting late in season brought in shoppers By Phil Wahba Jan 9 (Reuters) - U.S. retail sales rose 2.5 percent during the 2012 holiday season, helped by a late surge in shopping trips right before Christmas, retail tracking firm ShopperTrak said on Wednesday, the latest sign that retailers had a solid but not great season. ShopperTrak, which monitors the number of people walking into stores across the United States, said f