BUG: groupby.apply drops the group key

[pyarchana]

No description provided.

[precedent-bot]

From pandas-dev/pandas review history

Nobody asked me. I read the files this pull request changes and found three conventions the project has settled before, each linked to where it was settled.

• The first line of a docstring should be a single line summary.
  Established in a correction by pyarchana and pandas-dev/pandas#62880.

  Raised because it changes pandas/core/groupby/groupby.py.

  This helps maintain consistency and clarity in documentation across the project.

• Always include a whatsnew entry in the appropriate file for any bug fixes or new features.
  Established in pandas-dev/pandas#64119 and #61985.

  Raised because it changes doc/source/whatsnew/v3.0.0.rst.

  This helps keep the documentation up to date and informs users of changes.

• Use the code-block directive for demonstrating old behavior and the ipython directive for newly introduced behavior in the whatsnew files.
  Established in pandas-dev/pandas#63310 and #50626.

  Raised because it changes doc/source/whatsnew/v3.0.0.rst.

  This approach helps maintain clarity and accuracy in documenting changes across different versions of pandas.

If this is wrong or out of date, a maintainer can say so: reply @precedent <the convention as it actually is> and I will hold the correction from then on.