BUG: Grouper specified by key not regarded as in-axis

[rhshadrach]

Currently using as_index=False only includes in-axis groupers (see #49519). Defining a Grouper by the key argument is in-axis, but not regarded as such.

df = pd.DataFrame({'a': [1, 1, 2], 'b': [1, 1, 2], 'c': [3, 4, 5]})
gb = df.groupby([pd.Grouper(key='a'), pd.Grouper(key='b')], as_index=False)
result = gb.sum()
print(result)
#    c
# 0  7
# 1  5

gb2 = df.groupby(['a', 'b'], as_index=False)
result2 = gb2.sum()
print(result2)
#    a  b  c
# 0  1  1  7
# 1  2  2  5

The 1st case above should match the output of the 2nd case.