**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 if a a dagrun will not be scheduled if max_dag_runs
        has been reached and dagrun_timeout is not reached
        Test if a a dagrun would be scheduled if max_dag_runs has
        been reached but dagrun_timeout is also reached
        """
        with dag_maker(
            dag_id="test_scheduler_verify_max_active_runs_and_dagrun_timeout",
            start_date=DEFAULT_DATE,
            max_active_runs=1,
            processor_subdir=TEST_DAG_FOLDER,
            dagrun_timeout=datetime.timedelta(seconds=60),
        ) as dag:
            EmptyOperator(task_id="dummy")
        self.scheduler_job = SchedulerJob(subdir=os.devnull)
        self.scheduler_job.dagbag = dag_maker.dagbag
        session = settings.Session()
        orm_dag = session.get(DagModel, dag.dag_id)
        assert orm_dag is not None
        self.scheduler_job._create_dag_runs([orm_dag], session)
        self.scheduler_job._start_queued_dagruns(session)
        drs = DagRun.find(dag_id=dag.dag_id, session=session)
        assert len(drs) == 1
        dr = drs[0]
        assert orm_dag.next_dagrun_create_after is None
        
        assert isinstance(orm_dag.next_dagrun, datetime.datetime)
        assert isinstance(orm_dag.next_dagrun_data_interval_start, datetime.datetime)
        assert isinstance(orm_dag.next_dagrun_data_interval_end, datetime.datetime)