[AIRFLOW-1492] Add metric for task successes/failures

[saguziel]

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

@aoen @bolkedebruin

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-1492

Description

• Here are some details about my PR, including screenshots of any UI changes: Adds two gauges

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason: None needed, just emits metric

Commits

• 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":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[aoen]

success->successes (and same with failure) to be consistent with the other metric. Even better ti_successes since tasks are abstract.

[codecov-io]

Codecov Report

Merging #2504 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2504      +/-   ##
==========================================
+ Coverage   69.98%   69.99%   +<.01%     
==========================================
  Files         146      146              
  Lines       11446    11448       +2     
==========================================
+ Hits         8011     8013       +2     
  Misses       3435     3435

Impacted Files	Coverage Δ
airflow/models.py	87.32% <100%> (+0.01%)	⬆️

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[Acehaidrey]

A comment about statsd stats in Airflow in general they are incorrect some are incorrect. Was wondering will there be any priority to fix those stats? AIRFLOW-774 is what I'm referencing.

[aoen]

I don't think that they are incorrect in general, there is a bug with that specific metrics (dagbag_size/collect_dags) you mentioned but I'm not aware of any other ones with issues. The failures/successes metric should work since it can be summed across all processes.

[Acehaidrey]

Hey @aoen thanks for the response! Okay that's good to know, stats that are aggregated should be correct across all processes. I think you're right it was only those 3 stats in this ticket that we were seeing incorrectly

[aoen]

LGTM