Support for arbitrary times of now

[keithknott26]

Is there a way go-metrics could support arbitrary times of Now? Similar to the implementation of https://github.com/mathpl/go-timemetrics?

I need to be able to parse through a log file and then capture metrics out of the data stream, reading and updating the Now value as the stream progresses forward in time.

If this is possible, perhaps I've just missed how to implement it using the existing code?