metrics: Improve metrics_client usage docs.

BUG=chromium:393334
TEST=unit tests passed

Change-Id: Iaf6b75cba2fddce5ea5f770d227ed5df70c9bed8
Reviewed-on: https://chromium-review.googlesource.com/213471
Reviewed-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Thiemo Nagel <tnagel@chromium.org>
Tested-by: Thiemo Nagel <tnagel@chromium.org>
1 file changed
tree: e0558ababe5133db391a95f3d3df0710cbb3c179
  1. metrics/