metrics: Creates |Pause| and |Resume| methods for timers.

BUG=chromium:244589
TEST=unittest

Change-Id: I54e6dd7f5a8eb08ff42b1ee0e60df5cc895d0819
Reviewed-on: https://gerrit.chromium.org/gerrit/56849


Reviewed-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Wade Guthrie <wdg@chromium.org>
Tested-by: Wade Guthrie <wdg@chromium.org>
Commit-Queue: Wade Guthrie <wdg@chromium.org>