I would like to tile over the face of my fireplace insert. I am planning to use Marble (Forrest Green I think) tiles. I would like to glue them directly to the metal. My thought was to first sand the metal with a coarse grit paper to give a rough surface for the glue to adhere to. I do have some concerns with the heat from the fireplace causing problems. I am looking for recommendations as to whether or not this is a good idea, and if there is a preferred type or brand of glue that I should use.
Thanks!

Not going to work. The metal will expand and contract enough to break the bond between the tile and metal.

The best you can do is to rivet a wire mesh to the metal to increase your chances of a bond. But its a risk as the metal will still expand and contract.

Another concern is that the metal is designed to give off some heat. This prevents the rest of the fireplace from overheating where you do not want it to. By covering the face with another material such as stone/tile this may cause a fire hazard as the heat may not be given off as readily.

Bottom line, I would not do this.