Extensibility of DocValueSkippers

Hi folks,

We have a few open PRs adding new data to the DocValuesSkipper interface =
(eg https://github.com/apache/lucene/pull/15993, =
https://github.com/apache/lucene/pull/15737), and other open issues =
discussing adding more (https://github.com/apache/lucene/issues/15884). =
We also have some ideas here at elastic for other bits of information =
that would be useful in highly specific circumstances but not really in =
the general case.  These all run into issues with backwards =
compatibility, and questions of how to reliably signal to clients what =
data is available for a given field and segment.

One idea I had that would make this a bit more pluggable, and allow =
Codecs to add additional block-based data without having to alter the =
base API too much, is to add a SkipType object which would be passed to =
the LeafReader like so:

T getDocValuesSkipper(SkipType<T extends DocValuesSkipper> type)

The codec would check the class of the SkipType and see if it knows how =
to return that information.  If yes, it returns an instance of T, if not =
it returns null.  The default type would be a Range<DocValuesSkipper>, =
which would return the basic DocValuesSkipper that we have now, but we =
can extend things with a Count or Cardinality type.  On the indexing =
side, the FieldInfo could record the SkipType so that the codec knows =
what metadata to generate.

Some of these bits of information are useful both as global metadata and =
as part of a skip block; some are only really relevant at the global =
level.  Tying into the work that Ignacio is doing in =
https://github.com/apache/lucene/issues/16052, the global metadata tends =
to be loaded at segment open time and so can be accessed cheaply without =
doing any IO, but because it is part of the general DocValuesSkipper =
object it can only be accessed by calling =
LeafReader.getDocValuesSkipper() which loads a bunch of extra data (and =
declares that it does IO via its throws clause).

We could add an intermediate object here, analogous to Points or Terms, =
called DocValues (or something similar, I know this is already a class =
with static helper methods on it); this would make the global min, max =
and docCount (and maybe cardinality) available without having to do any =
further IO, and the getSkipper() method could optionally be moved onto =
the intermediate object.

What do people think?

- Alan=