DataFrame.groupby doesn't preserve _metadata
I'm following the 0.25.3 [documentation](https://pandas.pydata.org/pandas-docs/stable/development/extending.html#subclassing-pandas-data-structures) on using _metadata.

```python
sdf = SubclassedDataFrame2(my_data_as_dictionary)
sdf.added_property = 'hello pandas'
df = sdf.groupby('mycategorical')[['myfloat1', 'myfloat2']].sum()
print(df.added_property)
```
The above output produces the result:
AttributeError: 'DataFrame' object has no attribute 'added_property'

**added_property** is _not_ being "passed to manipulation results" as described in the documentation.