[GitHub] [airflow] ashb opened a new pull request #6157: [AIRFLOW-774] Fix long-broken DAG parsing Statsd metrics

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ashb opened a new pull request #6157: [AIRFLOW-774] Fix long-broken DAG parsing Statsd metrics
URL: https://github.com/apache/airflow/pull/6157
 
 
   Make sure you have checked _all_ steps below.
   
   ### Jira
   
   - [x] https://issues.apache.org/jira/browse/AIRFLOW-774
   
   ### Description
   
   - [x] Here are some details about my PR, including screenshots of any UI changes:
     Since we switched to using sub-processes to parse the DAG files sometime
     back in 2016(!) the metrics we have been emitting about dag bag size and
     parsing have been incorrect.
   
     We have also been emitting metrics from the webserver which is going to
     be become wrong as we move towards a stateless webserver.
   
     To fix both of these issues I have stopped emitting the metrics from
     models.DagBag and only emit them from inside the
     DagFileProcessorManager.
   
     (There was also a bug in the `dag.loading-duration.*` we were emitting
     from the DagBag code where the "dag_file" part of that metric was empty.
     I have fixed that even though I have now deprecated that metric)
   
   ### Tests
   
   - [x] No, cos these are a bit hard to test usefully, but I have looked at the metrics emmited.
   
   ### Commits
   
   - [x] My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "[How to write a good git commit message](http://chris.beams.io/posts/git-commit/)":
     1. Subject is separated from body by a blank line
     1. Subject is limited to 50 characters (not including Jira issue reference)
     1. Subject does not end with a period
     1. Subject uses the imperative mood ("add", not "adding")
     1. Body wraps at 72 characters
     1. Body explains "what" and "why", not "how"
   
   ### Documentation
   
   - [x] In case of new functionality, my PR adds documentation that describes how to use it.
     - All the public functions and the classes in the PR contain docstrings that explain what it does
     - If you implement backwards incompatible changes, please leave a note in the [Updating.md](https://github.com/apache/airflow/blob/master/UPDATING.md) so we can assign it to a appropriate release

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