Scenario: Pym Tech, a leading U.S.-based technology manufacturer, is thriving under the leadership of Hank Pym, with its headquarters located on Treasure Island, San Francisco, California. Hank is currently training a regression model in a notebook to predict daily customer visits to Pym Swag outlets. To effectively track model performance, Hank must log a metric using MLflow at the end of each training epoch. Question: Which metric should Hank use to log model performance in MLflow?