Filter XCOM by key when calculating map lengths

[hinnefe2]

MappedOperator determines how many downstream task instances to create
by counting XCOMs associated with upstream mapped tasks. The current
implementation assumes each upstream mapped task has a single XCOM. When
upstream tasks push additional XCOMs beyond the default one then
MappedOperator will generate too many downstream task instances. This
change updates the XCOM counting query to only consider XCOMs with a key
matching the XCOM_RETURN_KEY constant.

close #24487
close #23792

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (flake8, mypy and type annotations). Our pre-commits will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it’s a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[ashb]

Change looks good - can you add to the tests please?

[hinnefe2]

Sure, I'll put a test or two in tests/models/test_mappedoperator.py unless somebody points out a better place.

[jedcunningham]

@hinnefe2, gentle ping. It'd be great to get this into 2.3.3. That test file is the right spot. If you have further questions/get stuck, feel free to ping us in #development on Slack!

[hinnefe2]

Thanks for the nudge. I'm having a little trouble getting the test to work as intended but I'll sort that out on Slack.

[uranusjr]

One styling nitpick, the logic looks good to me.

[uranusjr]

I pushed a couple of commits to resolve conflicts and update the test.

[uranusjr]

I’m going to make the call since this is straightfoward enough.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[hinnefe2]

🎉 Thanks for the help getting this across the finish line!