Not fun. That is essentially the same construction as a restroom, so let's hope there is no standing water and a big handle on the wall.
You're past just ear plugs and limiters. You've got to study speaker placement, as well as give your RTA a serious workout. Then, I'm sure since you've mentioned it's a metal band, I'm sure you've got two "tone chasers" who don't know squat about tone or chasing, but they have those amps cranked to 11 just in case.
I am getting called in to do consulting for the strip clubs that are being shut down and forced to convert to "legitimate" businesses. These rooms are hopeless. Install some good speakers and amps, and get a decent DJ in there, or rotate DJ's and have theme nights or something. A stage barely big enough to hold a strung out meth hooker is NOT going to suddently big enough to hold a band!