metrics: don't upload metrics when metrics are disabled

The uploader should only send metrics samples when the metrics are
enabled.
The uploader daemon is still started when the metrics are disabled so
that:
* When we enable the metrics, we don't require a restart of
  metrics_daemon to start uploading metrics.
* The metrics file is truncated periodically and avoid taking too much
  space on long running system with metrics disabled.

BUG=chromium:459636
TEST=unittests
TEST=`test_that -b gizmo gizmo platform_MetricsUploader` works
TEST=manual: uploader does not upload metrics if metrics are disabled.

CQ-DEPEND=CL:250980

Change-Id: I9f5da3457066a183c5791b5488e985b7ab13b6e1
Reviewed-on: https://chromium-review.googlesource.com/250822


Trybot-Ready: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Nathan Bullock <nathanbullock@google.com>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>