A 29-year-old woman at 29 weeks has gestational diabetes managed with dietary advice and exercise. For the past 2 weeks, her fasting capillary glucose concentrations have been 5.4–5.8 mmol/L (target less than 5.0), while all 2-hour postprandial values are below 6.7 mmol/L. She has an estimated glomerular filtration rate of 38 mL/min/1.73 m² (greater than 60) and no ketonuria. Which one of the following is the most appropriate next treatment?