PERF: groupby(...).__len__

[rhshadrach]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Enabled by #55738. Prior to that, ngroups was not reliable.

size = 100_000
df = pd.DataFrame(
    {
        "a": np.random.randint(0, 100, size),
        "b": np.random.randint(0, 100, size),
    }
)
%timeit len(df.groupby(["a", "b"]))

# 161 ms ± 962 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)    <-- main
# 4.96 ms ± 229 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)  <-- PR

[rhshadrach]

Stumbled into this because of the added test. This should have been removed by #57330

[mroeschke]

Thanks @rhshadrach