As overall health care costs continue to rise sharply, prescription drugs have emerged as a surprising exception. Annual inflation in drug costs is at the lowest rate in the three decades since the Labor Department began using its current method of tracking prescription prices. The rate over the last 12 months is 1 percent, according to the government’s latest data, released Wednesday. “The way the index is going, it looks like drug price increases are not going to be very painful this year,” said Daniel H. Ginsburg, a supervisory economist at the Bureau of Labor Statistics, where he is involved...