**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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"""Try to expand the ti, if needed.
            If the ti needs expansion, newly created task instances are
            returned as well as the original ti.
            The original ti is also modified in-place and assigned the
            ``map_index`` of 0.
            If the ti does not need expansion, either because the task is not
            mapped, or has already been expanded, *None* is returned.
            """
            if ti.map_index >= 0:  
                return None
            try:
                expanded_tis, _ = ti.task.expand_mapped_task(self.run_id, session=session)
            except NotMapped:  
                return None
            if expanded_tis:
                return expanded_tis