metrics: remove gflags dependency

We are switching to using chromeos/flag_helper.h instead to standardize the
code everywhere.

BUG=chromium:375017
TEST=`FEATURES=test emerge-panther metrics`
TEST=`cbuildbot --remote --hwtest -p 'chromiumos/platform2'
      falco-paladin` passes

Change-Id: Icd08f65fd639e82ac6fe1581c763d60a189db827
Reviewed-on: https://chromium-review.googlesource.com/221757


Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Steve Fung <stevefung@chromium.org>
Tested-by: Steve Fung <stevefung@chromium.org>