[AIRFLOW-4739] Add ability to specify labels per task with kubernetes executor config

[george-miller]

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-4739
  • 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).
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Description

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

I want to be able to add arbitrary labels to an airflow worker pod. For example, I want to specify this in the task definition:

run_this = PythonOperator(
task_id='print_the_context',
provide_context=True,
python_callable=my_sleeping_function,
executor_config={"KubernetesExecutor": {"labels":
  {"test": "label"}
}},
dag=dag,
)

And have my worker pod have the label test:label.

My main use case for this is for auditing. We audit our kubernetes cluster by tags, attributing cost based on how much resources the pod uses.

Tests

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

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

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #5376 into master will decrease coverage by <.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #5376      +/-   ##
==========================================
- Coverage   79.02%   79.02%   -0.01%     
==========================================
  Files         481      481              
  Lines       30197    30200       +3     
==========================================
+ Hits        23864    23865       +1     
- Misses       6333     6335       +2

Impacted Files	Coverage Δ
...ontrib/example_dags/example_kubernetes_executor.py	0% <0%> (ø)	⬆️
airflow/kubernetes/worker_configuration.py	96.06% <100%> (+0.03%)	⬆️
airflow/executors/kubernetes_executor.py	66.14% <100%> (+0.08%)	⬆️
airflow/models/taskinstance.py	92.43% <0%> (-0.18%)	⬇️

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[pgagnon]

@dimberman