Transform function on metrics

This lets you report things like disk space in MB (you're monitoring
both boot and root, they both report in MB) but you want to display
some things in GB.

Can include a filter function in the metric to divide by 1024, and
now your MB are in GB.

Also:

* Use base when calling `parseInt`
* Small doc tweak to make it clear what the InfluxDB values are for.