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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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tests/jobs/test_scheduler_job.py
"""
        Test that the dag file processor does not call the sla_miss_callback when a
        notification has already been sent
        """
        session = settings.Session()
        
        sla_callback = MagicMock()
        
        
        test_start_date = days_ago(2)
        dag = DAG(
            dag_id='test_sla_miss',
            sla_miss_callback=sla_callback,
            default_args={'start_date': test_start_date, 'sla': datetime.timedelta(days=1)},
        )
        task = DummyOperator(task_id='dummy', dag=dag, owner='airflow')
        
        session.merge(TaskInstance(task=task, execution_date=test_start_date, state='success'))
        
        session.merge(
            SlaMiss(
                task_id='dummy',
                dag_id='test_sla_miss',
                execution_date=test_start_date,
                email_sent=False,
                notification_sent=True,
            )
        )