DocValues statistics helper

[Chitra R]

Hi,
      Great to see DocValuesStatsCollector is available in Lucene 6.4. It
helps to compute statistics on DocValues Field. Hope, it has been achieved
using GlobalOrdinalsCollector.

My doubt is

For statistics,
>
Global ordinals are computed only for query matching ordinals of a specific
> field which map per-segment ordinals to/from global ordinal space.
>

Thanks,
Chitra