BUG: DataFrameGroupBy.apply with as_index False column name
Example:

```
df = DataFrame({"A": [1, 2], "B": [3, 4]})
g = df.groupby("A", as_index=False)
result = g.apply(len)
print(result)
```

results in

```
   A  NaN
0  1    1
1  2    1
```

If we were to use `pandas.core.common.get_callable_name`, then we would be able to have the column name be "len" in this case, and presumably others.