[AIRFLOW-6041] Add user agent to the Discovery API client

[mik-laj]

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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-6041
  • 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:

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

[codecov-io]

Codecov Report

Merging #6636 into master will decrease coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #6636     +/-   ##
=========================================
- Coverage   83.71%   83.42%   -0.3%     
=========================================
  Files         672      672             
  Lines       37568    37767    +199     
=========================================
+ Hits        31450    31507     +57     
- Misses       6118     6260    +142

Impacted Files	Coverage Δ
airflow/gcp/hooks/base.py	96.98% <100%> (+0.03%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/executors/sequential_executor.py	47.61% <0%> (-52.39%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	88.39% <0%> (-10.22%)	⬇️
airflow/cli/commands/worker_command.py	31.48% <0%> (-8.06%)	⬇️
airflow/utils/sqlalchemy.py	86.44% <0%> (-5.09%)	⬇️
airflow/executors/__init__.py	63.26% <0%> (-4.09%)	⬇️
... and 13 more

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