Proposal: Introduce heartbeat metric document

We currently treat an instance as active if it is ingesting data. So if it receives traffic it will transmit transactions/spans/errors and/or if metrics is enabled it'll periodically transmit system metrics (cpu, memory).

However, it is theoretically possible for an instance to be active but not receiving any traffic, and metrics being disabled. In this case it'll be treated as not active (terminated).

This has implications on how [throughput for the instance is calculated](https://github.com/elastic/kibana/issues/89497).

**Question**
Would it make sense to have a "heartbeat" document that is broadcasted periodically (_Addendum: And is not possible to disable_)?

cc @felixbarny @axw