Reduce the number of calls to document() from LeafStoredFieldsLookup

[romseygeek]

LeafStoredFieldsLookup currently uses a SingleFieldsVisitor to load values, which
will be inefficient if a script references more than one stored field. This commit
refactors the lookup to remember all the fields that are requested, along with the
ability to pre-register fields so that the first lookup will also only read from disk
once.

Relates to #66256

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[romseygeek]

The idea here is that in future SourceLookup can depend on LeafStoredFieldsLookup to load its values, allowing us to consolidate loading source and stored fields together in a single place rather than spread across the Fetch phase.

[jtibshirani]

I still need to ponder the refactor in detail, but had some initial thoughts.

LeafStoredFieldsLookup currently uses a SingleFieldsVisitor to load values, which will be inefficient if a script references more than one stored field.

It's rare for scripts to access stored fields, and in general we hope to decrease reliance on stored fields (maybe eventually deprecating them?) So this refactor doesn't feel high priority in itself. However you mention that this is an incremental step towards having SourceLookup use a shared stored fields visitor. I don't yet see how this would work -- would you be able to describe the plan in more detail (or even show a quick prototype if you've tried it out?)

The idea here is that in future SourceLookup can depend on LeafStoredFieldsLookup to load its values,

I see these Leaf*Lookup classes as specifically designed for scripting. They make sure to implement Map<String, Object>, sometimes add an extra caching layer, etc. I wonder if we should keep script data structures separate from the general lookup case for flexibility/ clarity. The fact that SourceLookup is used in both cases has actually caused complexity, see this comment for example.

[mark-vieira]

@elasticmachine update branch

[anupamgogoi0907]

Hi Team,
Is the issue solved? I don't see any further comments.
Thanks.