A 48-year-old woman with hypertension and type 2 diabetes mellitus is prescribed metoprolol 100 mg daily. After 3 weeks, she reports persistent fatigue, inability to exercise, and a 2 kg weight gain. Her blood glucose (fasting) has increased from 110 mg/dL to 145 mg/dL. HR is 58/min (baseline 72/min), BP 128/82 mmHg. She is otherwise asymptomatic. What is the most appropriate next step in management?