Rework NumGC and PauseNs runtime metrics.

Fixes #41.

@majek is right that numGC not being updated breaks the PauseNs
histogram pretty badly.  What's more, the simplistic bridge between
runtime.MemStats and go-metrics was unable to cope with a couple of
obvious possibilities like wrapping PauseNs and more than 256 GCs since
the last metric update.