Translate LINQ CountBy

[manandre]

.NET 9.0 will introduce CountBy, which we could translate.

CountBy can be rewritten as follows:

_ = blogs.CountBy(b => b.Id);
_ = blogs.GroupBy(b => b.Id).Select(g => KeyValuePair.Create(g.Key, g => g.Count()));

This feature could be part of the epic #25570

[roji]

Thanks @manandre, makes sense! Added to the epic.