[AIRFLOW-5811] add metric for externally killed task count

[houqp]

Make sure you have checked all steps below.

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-XXX
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal (AIP).
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Description

We found this to be a very good signal to alert for any init container related issue. Our worker cluster was affected by a bug in EKS CNI plugin. This metric would have caught the issue automatically.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Looks like we don't test for stats?

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 (not including Jira issue reference)
  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"

Documentation

• 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 so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6466 into master will decrease coverage by 0.36%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6466      +/-   ##
==========================================
- Coverage   83.95%   83.58%   -0.37%     
==========================================
  Files         635      635              
  Lines       36657    36662       +5     
==========================================
- Hits        30774    30645     -129     
- Misses       5883     6017     +134

Impacted Files	Coverage Δ
airflow/jobs/scheduler_job.py	74.96% <100%> (+0.03%)	⬆️
airflow/operators/postgres_operator.py	0% <0%> (-100%)	⬇️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️
airflow/hooks/postgres_hook.py	94.73% <0%> (-1.76%)	⬇️
airflow/utils/sqlalchemy.py	91.52% <0%> (-1.7%)	⬇️
airflow/hooks/dbapi_hook.py	88.98% <0%> (-1.7%)	⬇️
... and 4 more

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