A 16oz monster has 160mg of caffeine. A cup of coffee has anywhere from 100-150 usually. It's not that big of a deal. The caffeine isn't necessarily the performance enhancer- the taurine and whatever else is in there probably does more. For caffeine to have a major positive effect on a runner, it would probably take more like 300mg. There are studies done on this with different amounts and the effects they have on performance, just use google.
Your coach shouldn't be giving it to you, and if he is buying it for you in order to enhance performance, then it is a big deal. But it's not a big deal if you purchase it and drink it. You will probably run faster in anything under 5k. Caffeine is perfectly legal up to a ridiculously high amount- an amount no normal person would ever consume before racing.