Watertown-based athenahealth Inc., a provider of online services to doctors and hospitals, is acquiring Epocrates Inc. of San Mateo, Calif., for $293 million to gain a leading iPhone app and expand in electronic records. Athenahealth will pay $11.75 a share, a 22 percent premium to Epocrates’s Friday closing price. The deal helps athenahealth, which processes insurance claims and patient payments, expand a business for software to prescribe drugs and keep records. It may also benefit from name recognition for Epocrates’s app, as of August used by 338,000 physicians to call up summaries on drugs.