Even if you did have a moon made completely out of carbon, it would not be completely made of diamond. You are neglecting the role of gravity here. You are expecting a highly refractive, transparent cosmic body which is not practically possible. Only the innermost layers would be composed of diamond, the outer layers would be buckminsterfullerene powder or huge layer of opaque graphite. Furthermore, the asteroids hitting the surface would be depositing a large amount of opaque dust on the surface which would further decrease it's role as a colossal lens.
A lens shaped moon-sized body is not practically possible due to gravity, which tends to squeeze things in a form where most matter can be compressed in a shape that has the least surface area. It happens to be a sphere. A lens shaped body has a much larger volume to surface area ratio. The sides of this lens (even if it happen to form) would be crushed in and after sufficient amount of time (a few hundreds of millions of years), it will crack into a spherical form we see today.
Even if you did have a colossal lens shaped body (extremely improbable) which tends to be made of transparent material (severely improbable) which also happens to be at 90° angle against a planet (next to impossible), still the chance of getting metal-boiling temperatures would be very low, because, as you know, the planet surface would have to be exactly (or a few hundred plus or minus) kilometers to the focal point of that colossal lens.