Fix metronome

[zehreken]

I have been having difficulties keeping time recently while using the builtin metronome. I thought that it was my bad but now I think there is a bug with the metronome. It does not sound right.

[zehreken]

The metronome was buggy. I first implemented a new version using std::time but after discussing it on Rust Audio on discord, I was advised not to use std::time. It still uses samples to keep the time and works fine.