BUG: Keep column level names during resample nunique (#23222)
- [X] closes #23222
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

nunique is the only one I found in a quick search which manifested the problem.
Hello @dsm054! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/groupby/generic.py`](https://github.com/dsm054/pandas/blob/b59352406dbaf6b78f4778662b36a75f5beea870/pandas/core/groupby/generic.py) !

 - There are no PEP8 issues in the file [`pandas/tests/groupby/test_function.py`](https://github.com/dsm054/pandas/blob/b59352406dbaf6b78f4778662b36a75f5beea870/pandas/tests/groupby/test_function.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_resample.py`](https://github.com/dsm054/pandas/blob/b59352406dbaf6b78f4778662b36a75f5beea870/pandas/tests/test_resample.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23645?src=pr&el=h1) Report
> Merging [#23645](https://codecov.io/gh/pandas-dev/pandas/pull/23645?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/71cd230f3ce3e29760b31dfcfa391cd2c4371842?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23645/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23645?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23645      +/-   ##
==========================================
+ Coverage   92.24%   92.24%   +<.01%     
==========================================
  Files         161      161              
  Lines       51314    51318       +4     
==========================================
+ Hits        47335    47339       +4     
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <100%> (ø)` | :arrow_up: |
| #single | `42.31% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23645?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23645/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `87.09% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23645/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.81% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/23645/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.03% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/23645/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.7% <0%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23645?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23645?src=pr&el=footer). Last update [71cd230...8dd1d36](https://codecov.io/gh/pandas-dev/pandas/pull/23645?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 if you can merge master @dsm054 can you merge master Closing as stale. Ping if you'd like to continue Revived in #25469