export_to_gcloud: Add a metric for calls

Add a metric for calls to export_to_gcloud from cbuildbot stages.

BUG=chromium:709653
TEST=commands_unittest passes.

Change-Id: I485657cdeaa45ed92728f321b445ae0de24828d7
Reviewed-on: https://chromium-review.googlesource.com/471833
Commit-Ready: Paul Hobbs <phobbs@google.com>
Tested-by: Paul Hobbs <phobbs@google.com>
Reviewed-by: David Riley <davidriley@chromium.org>
4 files changed