sysutils/mtail: New Port

mtail is a tool for extracting metrics from application logs to be
exported into a timeseries database or timeseries calculator for
alerting and dashboarding.

It fills a monitoring niche by being the glue between applications that
do not export their own internal state (other than via logs) and
existing monitoring systems, such that system operators do not need to
patch those applications to instrument them or writing custom extraction
code for every such application.