metrics: Document uploader functionality

Metrics uploader can be activated with the metrics_uploader use flag, update
README to reflect that.
Also fix the section about activating metrics.

BUG=None
TEST=None

Change-Id: I2c6d0abe6536eb419c5b49f149b6d3f097670325
Reviewed-on: https://chromium-review.googlesource.com/216631


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>