(FACT-2927) Add method determine_metric_prefix
(FACT-2927) Add method determine_metric_prefix

This makes rubocop happy (because it reduces AbcSize for method
hertz_to_human_readable). Also, the new (private) method could
potentially be reused for other things than speed.

puppetlabs/facter,puppetlabs/facter,puppetlabs/facter