Metrics replaces the live sample, and sending one IS A HEARTBEAT: the server stamps the time and appends the point to the fleet series. Sending an all-zero sample CLEARS the heartbeat — the machine goes back to having no liveness fact at all, and its stored status is taken at face value again.
Status sets operator INTENT: online | offline | draining. Draining is how a machine is taken out of dispatch without ending what is already on it. What comes back may still read offline, because the heartbeat outranks the intent.