hmm, after looking at the code closer again (it has been a while), I think the docs are just wrong. For discrete faceting you cannot use numeric fields. Hence why you see the behavior you describe. Unfortunately, it is really a precondition with the current way we do faceting, namely using a custom Collector. We are looking at different ways of doing it, but for now I think the best we can do is to sync the doc.
Thanks for pointing this out.

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