**Description**


It would be great if the metrics we publish in Airflow change to include TAGs for "variable" parts of those (such as dag_id, task_id, pool name) rather than being part of the metrics.

See here: https://airflow.apache.org/docs/1.10.12/metrics.html, https://airflow.readthedocs.io/en/latest/logging-monitoring/metrics.html  - those variable parts should be tags.

We might consider to change those metrics by default in 2.0 and possibly introduce a backport in 1.10.13 to allow people to migrate.


**Use case / motivation**

Having the variable parts of the metrics makes it really difficult to aggregate the metrics. Seems that statsd python library does not support tags, but there are possible solutions by installing extensions:

https://stackoverflow.com/questions/49852654/sending-statsd-metrics-with-tags-from-python-client

**Related Issues**

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"""
        Test that if a dag run exists, scheduler._create_dag_runs does not raise an error.
        And if a Dag Run does not exist it creates next Dag Run. In both cases the Scheduler
        sets next execution date as DagModel.next_dagrun
        """
        with dag_maker(
            dag_id="test_scheduler_create_dag_runs_check_existing_run",
            schedule=timedelta(days=1),
        ) as dag:
            EmptyOperator(
                task_id="dummy",
            )
        session = settings.Session()
        assert dag.get_last_dagrun(session) is None
        dag_model = dag_maker.dag_model
        
        assert dag_model.next_dagrun == DEFAULT_DATE
        dagrun = dag_maker.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=dag_model.next_dagrun,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
            external_trigger=False,
            session=session,
            creating_job_id=2,
        )
        session.flush()
        assert dag.get_last_dagrun(session) == dagrun
        self.scheduler_job = SchedulerJob(subdir=os.devnull, executor=self.null_exec)