**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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"""This tests that queued dagruns stops creating once max_active_runs is reached"""
        with dag_maker(max_active_runs=10) as dag:
            EmptyOperator(task_id="mytask")
        session = settings.Session()
        self.scheduler_job = SchedulerJob(subdir=os.devnull)
        self.scheduler_job.executor = MockExecutor()
        self.scheduler_job.processor_agent = mock.MagicMock()
        self.scheduler_job.dagbag = dag_maker.dagbag
        session = settings.Session()
        orm_dag = session.get(DagModel, dag.dag_id)
        assert orm_dag is not None
        for _ in range(20):
            self.scheduler_job._create_dag_runs([orm_dag], session)
        drs = session.query(DagRun).all()
        assert len(drs) == 10
        for dr in drs:
            dr.state = State.RUNNING
            session.merge(dr)
        session.commit()
        assert session.query(DagRun.state).filter(DagRun.state == State.RUNNING).count() == 10
        for _ in range(20):
            self.scheduler_job._create_dag_runs([orm_dag], session)
        assert session.query(DagRun).count() == 10
        assert session.query(DagRun.state).filter(DagRun.state == State.RUNNING).count() == 10
        assert session.query(DagRun.state).filter(DagRun.state == State.QUEUED).count() == 0
        assert orm_dag.next_dagrun_create_after is None
    def test_runs_are_created_after_max_active_runs_was_reached(self, dag_maker, session):