Fix metric names

Created by: hacklschorsch

Fixes #95 (closed).

@exarkun What do I do about materialization? I just changed the materialized nix file by hand, that made it work. I only changed three strings, why is this so complicated

@exarkun Something about shell.nix is funky - it currently requries a ./default.nix that isn't there. Maybe it's supposed to require ./nix/default.nix instead?