One stock market valuation metric is flashing a red warning light, but these three stocks may still be bargains nevertheless.A stock market bubble happens when the value of stocks grows faster than the value of the businesses they represent. And there's good reason to believe investors are in one right now.Years ago, famed investor Warren Buffett proposed a metric to measure stock valuations generally -- take the entire value of the U.S. stock market and divide it by U.S. GDP. Today, investors call this met ...