Compute facets counts from previously indexed {@link SortedSetDocValuesFacetField}, without require a separate taxonomy index. Faceting is a bit slower (~25%), and there is added cost on every {@link IndexReader} open to create a new {@link SortedSetDocValuesReaderState}. Furthermore, this does not support hierarchical facets; only flat (dimension + label) facets, but it uses quite a bit less RAM to do so.
NOTE: this class should be instantiated and then used from a single thread, because it holds a thread-private instance of {@link SortedSetDocValues}.
NOTE:: tie-break is by unicode sort order
@lucene.experimental
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