Cava is opening restaurants at a rapid clip, which should be good for growth. Just make sure you keep an eye on one certain metric in the meantime.Restaurant brands come and go, but when a concept is new and attractive it can lead to exciting stories on Wall Street. That's basically what's going on today with Cava Group (CAVA 10.50%), a Mediterranean fast-casual restaurant that's drawing in customers in huge numbers. The next three years are likely to see the company grow at a rapid clip.But investors need ...