Refactor metrics_utils from select_to_speak.

This moves all metrics related functionality into a separate class,
and is a pure refactor.

Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Idba2a55655c3e67476500618e6c324a10cfa0b37
Reviewed-on: https://chromium-review.googlesource.com/1141115
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576319}