I don't think that anyone saw this coming three years ago.On Sept. 5, S&P Global announced that advertising-technology company AppLovin (APP 0.79%) would be added to the S&P 500 index later this month -- a milestone in AppLovin's publicly traded journey that was unimaginable just three years ago.AppLovin makes money by helping to get its customers' apps -- usually mobile video games -- discovered and monetized. But three years ago, its business results were mediocre. In fact, it almost seemed like managemen ...