add length (or nrow) to describe

[bkamins]

There are two natural cases in describe that are currently hard:

1. get the number of rows;
2. get the number of non-missing rows.

@nalimilan - would you have any preference here?

[nalimilan]

Computing the number of rows doesn't sound too useful as it's the same for all columns. But indeed computing the number of non-missing entries sounds useful and even if length(::SkipMissing) (https://github.com/JuliaLang/julia/issues/42672) was implemented it could make sense to support e.g. :nonmissing or :nnonmissing for clarity.

[bkamins]

OK - let us go for :nnonmissing for consistency