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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
from airflow.exceptions import AirflowException, SerializedDagNotFound, TaskNotFound
from airflow.executors.executor_loader import UNPICKLEABLE_EXECUTORS
from airflow.jobs.base_job import BaseJob
from airflow.models import DAG, DagModel, SlaMiss, errors
from airflow.models.dagbag import DagBag
from airflow.models.dagrun import DagRun
from airflow.models.serialized_dag import SerializedDagModel
from airflow.models.taskinstance import SimpleTaskInstance, TaskInstanceKey
from airflow.stats import Stats
from airflow.ti_deps.dependencies_states import EXECUTION_STATES
from airflow.utils import timezone
from airflow.utils.callback_requests import (
    CallbackRequest,
    DagCallbackRequest,
    SlaCallbackRequest,
    TaskCallbackRequest,
)
from airflow.utils.dag_processing import AbstractDagFileProcessorProcess, DagFileProcessorAgent
from airflow.utils.email import get_email_address_list, send_email
from airflow.utils.log.logging_mixin import LoggingMixin, StreamLogWriter, set_context
from airflow.utils.mixins import MultiprocessingStartMethodMixin
from airflow.utils.retries import MAX_DB_RETRIES, retry_db_transaction, run_with_db_retries
from airflow.utils.session import create_session, provide_session
from airflow.utils.sqlalchemy import is_lock_not_available_error, prohibit_commit, skip_locked, with_row_locks
from airflow.utils.state import State
from airflow.utils.types import DagRunType
TI = models.TaskInstance
DR = models.DagRun
DM = models.DagModel
class DagFileProcessorProcess(AbstractDagFileProcessorProcess, LoggingMixin, MultiprocessingStartMethodMixin):