Stats units

[deadtrickster]

I think currently units used (in golang, last time I checked) for (self) documentation and in exporters (again, docs?). I think we can do better:

• Automatic units conversion. If a measure uses kibi and a view wants mibi, we can convert, no problem;
• Special time handling (as in prometheus.erl). When a unit is the time unit, aggregations expect native time units and do conversion when exporting (a view can still override).

[hauleth]

My idea for that is to support allow oc_stat:record/2-3 to accept tuple in form of {Unit, Value} where Unit can be any atom. Then we could have option to register "unit conversion module" that would have 1 function Module:convert(Value, From, To) :: {ok, Value} | error. Default module would always pass all values if the unit match and fail when there is mismatch. This would provide interface flexible enough for converting between any units, while keeping it quite simple as well.