[Heartbeat] Write summary document when termination signal is received

**Describe the enhancement:**

Find if there is a way to hook in to the Termination Signals in beats https://github.com/elastic/elastic-agent-libs/blob/d95aa61a0d89cbc5d30b8ba80928616e4a879827/service/service.go#L52 to write `heartbeat/summary` documents when a Browser Monitor run is missed.

The summary document `heartbeat/summary` should introduce a new monitor status `skipped` and the reason along with the error. The scope of this can be limited to just for `run_once` mode.

**Describe a specific use case for the enhancement or feature:**

Its pretty common for the Heartbeat container to get terminated when run as a Private Location or inside the hosted Synthetics infrastructure during upgrades or regular maintenance work.