[FEATURE]: Use fast path when grouping by unique monotonic decreasing variable

[dcherian]

Is your feature request related to a problem?

See https://github.com/pydata/xarray/pull/6213/files#r795716713

We check whether the by variable for groupby is unique and monotonically increasing. But the fast path would also apply to unique and monotonically decreasing variables.

Describe the solution you'd like

Update the condition to is_monotonic_increasing or is_monotonic_decreasing and add a test.

Describe alternatives you've considered

No response

Additional context

No response

[JoelJaeschke]

Hi @dcherian, I took a look at this issue and would appreciate your feedback :)