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**Apache Airflow version**: 2.0.0 but applicable to master

**Environment**: Running on ECS but not relevant to question


**What happened**: I am not seeing the metric dagrun.schedule_delay.<dag_id> being reported.  A search in the codebase seems to reveal that it no longer exists.   It was originally added in https://github.com/apache/airflow/pull/5050.

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<!-- What do you think went wrong? -->
I suspect either:
1.  This metric was intentionally removed, in which case the docs should be updated to remove it.
2. It was unintentionally removed during a refactor, in which case we should add it back.
3. I am bad at searching through code, and someone could hopefully point me to where it is reported from now. 

**How to reproduce it**:
https://github.com/apache/airflow/search?q=schedule_delay


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airflow/jobs/scheduler_job.py
"""
        This function is where the main scheduling decisions take places. It:
        - Creates any necessary DAG runs by examining the next_dagrun_create_after column of DagModel
          Since creating Dag Runs is a relatively time consuming process, we select only 10 dags by default
          (configurable via ``scheduler.max_dagruns_to_create_per_loop`` setting) - putting this higher will
          mean one scheduler could spend a chunk of time creating dag runs, and not ever get around to
          scheduling tasks.
        - Finds the "next n oldest" running DAG Runs to examine for scheduling (n=20 by default, configurable
          via ``scheduler.max_dagruns_per_loop_to_schedule`` config setting) and tries to progress state (TIs
          to SCHEDULED, or DagRuns to SUCCESS/FAILURE etc)
          By "next oldest", we mean hasn't been examined/scheduled in the most time.
          The reason we don't select all dagruns at once because the rows are selected with row locks, meaning
          that only one scheduler can "process them", even it is waiting behind other dags. Increasing this
          limit will allow more throughput for smaller DAGs but will likely slow down throughput for larger