Add heartbeat metric for DAG processor

[jedcunningham]

Description

The scheduler emits a scheduler_heartbeat, which makes it easy to determine if the scheduler is up and running. We should add an equivalent for the DAG processor.

Use case/motivation

No response

Related issues

No response

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[shalberd]

thank you, @jedcunningham @rawwar @potiuk nice catch in main (commit date-wise), though it did not make it into release notes, it seems. https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#airflow-2-10-3-2024-11-04
Can you confirm this change is in release 2.10.3 or later? Or is there a reason it hasn't been added to 2.10.x yet? I mean, dag processor has been optionally separate for a long time now.