system_api: service_constants.h: add MetricsEvent

This adds constants for the MetricsEventService.
Ideally this would have gone in with the addition
of metrics_event.proto, but I had my chrome and cros
sources out of sync.

BUG=chromium:729335
TEST=chromium compiles (cros clones these in Rust code)

Change-Id: I2407619196dd6fe75e3143caa932bb443ad5fffc
Reviewed-on: https://chromium-review.googlesource.com/1204910


Commit-Ready: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>