[C++][Dataset] Add ParquetFileFragment::num_row_groups property

[asfimport]

From dask/dask#6534 (comment), comment by @rjzamora:

it would be great to have access the total row-group count for the fragment from a num_row_groups attribute (which pyarrow should be able to get without parsing all row-group metadata/statistics - I think?).

One question is: does this attribute correspond to the row groups in the parquet file, or the (subset of) row groups represented by the fragment?
I expect the second (so if you do SplitByRowGroup, you would get a fragment with num_row_groups==1), but this might be a potential confusing aspect of the attribute.

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Ben Kietzman / @bkietz

PRs and other links:

• GitHub Pull Request #8317

_{Note: This issue was originally created as ARROW-10134. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
@jorisvandenbossche Maybe in Python ParquetFileFragment could have a property named whole_file_fragment or so, which would be set on child/subset fragments by split_by_row_group and other subsetting methods. f.whole_file_fragment.num_row_groups is then unambiguously the row groups of the whole file. This would also allow using f.whole_file_fragment == f to check for subselection

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
For checking if it is a subselection of row groups, we could also have something like a IsSubset method? That's maybe simpler as a whole_file_fragment and covers an important example use case you gave for whole_file_fragment

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 8317
#8317