Otherwise, when the step completes, it won't be able to close
files used for stdout/stderr, since the metrics process is still
running.

Bug: 1206429
Change-Id: Id88578cbd9dab070e07f92e545ec137245566d16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2877608
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>