Update metrics.README.md and add a metrics_version field.

Bug: None
Change-Id: I17d9032bd787af2a6fc49f80d59e05152d5223b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1406256Reviewed-by: Andy Perelson <ajp@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>