Statistics for list feature
Return distribution of lengths for lists (independently of what's inside).
_Note_: empty list is not counted as `na` value (so, not increment `na_count`), it is len=0.

I haven't done a proper testing of speed in an isolated environment (because i'm fighting with dev dockers locally, idk why), but a worker running locally without a container processes this dataset https://huggingface.co/datasets/polinaeterna/benchmark very fast (much more time was spend on download). So I assume it wouldn't be too much time to recompute.  

Also I tried to refactor the code for computation a bit, it can be better, feel free to suggest the way to improve it :)

~~Before merge I wanted to merge [this PR](https://github.com/huggingface/datasets-server/pull/2469) - to better track stats duration for bigger datasets, but I'm not sure I'm doing it right so would appreciate if someone who knows Prometheus take a look.~~ - merged

\+ this PR fixes this issue https://github.com/huggingface/datasets-server/issues/1843
Statistics for list feature: Return distribution of lengths for lists (independently of what's inside).
_Note_: empty list is not counted as `na` value (so, not increment `na_count`), it is len=0.

I haven't done a proper testing of speed in an isolated environment (because i'm fighting with dev dockers locally, idk why), but a worker running locally without a container processes this dataset https://huggingface.co/datasets/polinaeterna/benchmark very fast (much more time was spend on download). So I assume it wouldn't be too much time to recompute.  

Also I tried to refactor the code for computation a bit, it can be better, feel free to suggest the way to improve it :)

~~Before merge I wanted to merge [this PR](https://github.com/huggingface/datasets-server/pull/2469) - to better track stats duration for bigger datasets, but I'm not sure I'm doing it right so would appreciate if someone who knows Prometheus take a look.~~ - merged

\+ this PR fixes this issue https://github.com/huggingface/datasets-server/issues/1843