In the future, we would do better to handle/generate derived metrics with some plugin or config setup, as I think `space_used_percentage = space_used / (space_free + space_used)` would be more obvious / visible even if you had to write it in the config.  This starts to sound like a Complex Event Processor, but obviously if one had such a thing, they wouldn't need this sort of redundant output.  ;-)  (Though there is something to be said for doing math/rollup on-node and sending fewer metrics down the wire.)