added exometer_folsom_monitor

Some more testing at scale needed, but the idea is to allow monitoring of the creation of metrics in folsom (using the folsom API), and shadowing them with exometer metrics. This is to make it possible to have e.g. some legacy code that uses folsom directly, and still make use of exometer and exometer reporting.

https://github.com/Feuerlabs/exometer/blob/uw-folsom-monitor/doc/exometer_folsom_monitor.md