NeuroMetrix Inc., a Waltham medical device maker, said its wearable technology to treat chronic pain has been cleared by the Food and Drug Administration for over-the-counter sales. The Sensus Pain Management System (right), launched last year, uses electrical stimulation of nerves. The FDA decision lets a consumer buy the device without a doctor's prescription. NeuroMetrix also develops technology and tests for nerve diseases and sleep disorders. A major focus is diabetic neuropathies, which can trigger foot ulcers and amputations.