DataFrame groupby does not support Grouper objects #21594
Description
pandas has the concept of a Grouper, which can be passed to groupby to provide additional metadata. Currently we don't support these in the DataFrame API, we assume all arguments are just labels.
We should explicitly reject these for now, then add support for them.
Imported from Jira BEAM-14162. Original Jira may contain additional context.
Reported by: bhulette.