ENH: Preserve attrs when converting to pandas dataframe

Is your feature request related to a problem? Please describe.

import xarray

xds = xarray.DataArray([1], name="a", dims="a", attrs={"long_name": "Description about data"})

xds.attrs

Output:

{'long_name': 'Description about data'}

xds.to_dataframe().a.attrs

Output:

{}

Describe the solution you'd like
It would be nice if the attributes of the DataArray were preserved in each pandas.Series and the attributes of each Dataset were preserved on the pandas.Dataframe

Additional context

Things to be wary about is that it the pandas documentation says the attrs is experimental.

Start at the DataArray.to_dataframe conversion and the corresponding Dataset conversion entry point. Check how pandas Series.attrs and DataFrame.attrs are handled, then add coverage showing DataArray attributes on the resulting Series and Dataset attributes on the resulting DataFrame. Done means the requested attributes are preserved without relying on undocumented pandas behavior.