fetch DAG stats from DagRun instead of DagStat

[sockeye44]

fix for #46

[elephantum]

@sockeye44 thanks for the PR!

Question: do I understand correctly that this change breaks compatibility with Airflow versions before 1.10.3?

[sockeye44]

As I understand it, it can break compatibility with some of the oldest versions, but the underlying tables/fields have been added years ago:

• Migration for dag_run table, creation date: 2015-10-27 (https://github.com/apache/airflow/blob/master/airflow/migrations/versions/1b38cef5b76e_add_dagrun.py)
• Migration for dag table, creation date: 2015-08-18 (https://github.com/apache/airflow/blob/master/airflow/migrations/versions/e3a246e0dc1_current_schema.py)

[elephantum]

Yes, this change breaks compatibility in a silent way: it does not crash, but returns no metrics on 1.10.0.

Let's discuss what can be done about that.

Maybe we can create special branch that supports 1.10.3(rc2)+ and leave current version with note of incompatibility with 1.10.3

[elephantum]

I will merge this PR into separate branch airflow-1.10.3 for now.

When 1.10.3 gets released, we will drop compatibility with previous versions and make a new release of airflow-exporter.