Define heartbeat events semantic conventions

[pellared]

Propose new conventions

The heartbeat events could help detect cases where the service is hanging or crashed.

Related discussion: open-telemetry/opentelemetry-dotnet-contrib#2704 (comment)

[pellared]

From 2025-04-22 Spec SIG meeting:

@trask: It seems to be redundant with

semantic-conventions/docs/system/process-metrics.md

Line 341 in 69fae28

### Metric: `process.uptime`

@jsuereth: We likely do not need to be redundant.

@lmolkova: It would be to good to follow-up on Semantic Convention SIG meeting.