Well, I don't know of a site that directly addresses the structure of a metal song, but I do have some advice.

1. Most Metal songs are structured in a way that I like to call a "Popular Music Structure." There are all small variances on this type, however since vocals usually play a large role in this structure's music, they all are similarly related to each other in some way.

First, lets address the melody. The melody of the song is the "catch" or the "hook" and is the first thing that people recognize when a song comes on. In metal this could be a introductory riff or the melody of the lyrics.

Next, we have harmony. Since metal usually deals with catchy or interesting rhythmic patterns, this is also a point which your focus should be concentrated towards. However, since most voicings of chords, besides power chords, sound like unneccessary noise (with tons of gain and distortion), the most simplistic chord forms are usually used to support the melody.

Lastly we have the solo. Since most music that deals within the "Popular Music Structure," does not change keys or time signatures, it is fairly easy to understand and improv over. It's really up to the artist to come up with a great solo, however some of the best are usually developments off of the original melody. People like to hear something they recognize, but with twists and in metal, fast virtuosic technique added to it.

Anyway, now that I have described some of the parts of this structure, I will give you the "formula."

Intro -> Verse1 -> Chorus->Verse2->Chorus->Solo->Chorus->End

Now remember, there can be many extensions or reductions to this basic formula, however this is the most basic structure of Metal songs that sound coherant and are easily recognizable by most people. Its also up to you to decide what lyrics/harmonies are needed in the verses and chorus. Just remember that the most memorable lyrics/Melody or the "catch" is in the chorus.

I hope this helped. Peace.