math/rubygem-mtrc: new port - minimal metric aggregation library

Mtrc (short for metric) is a small library to accumulate metrics and
extract basic statistics, such as histograms and percentiles, from them.
It takes samples, and provides tick counters, percentiles, and sorted
datasets. This can be used for profiling repeated events such as
application requests or message queue arrival rates.

WWW: https://github.com/aphyr/mtrc

Reviewed by: romain
Approved by: jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D17710