A 55-year-old woman with type 2 diabetes mellitus attends for her annual review. Her latest HbA1c is 59 mmol/mol (target ≤53 mmol/mol). She is currently taking metformin 1g BD. She has a BMI of 32 kg/m$^2$ and established cardiovascular disease (previous myocardial infarction). According to NICE NG28, what is the most appropriate second-line agent to add?