send_monitoring_event: added --event-mon-event-timestamp
send_monitoring_event: added --event-mon-event-timestamp

BUG=473796

Review URL: https://codereview.chromium.org/1191423002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra