Index-time scripted fields to be fetched from doc values

[javanna]

Certain field types support index-time scripts. When a script is provided for them, values for such fields are not accepted in incoming documents. Instead, the script is executed at index time and the result of the script is indexed. There is no trace of the resulting value in _source, but such calculated fields can be queried, aggregated on etc.

The described calculated fields can also be retrieved as part of the fetch phase, using the fields API or docvalue_fields. The fetch fields api is though re-executing the script at fetch time as opposed to retrieving the value that is in the index. That causes discrepancy between indexing, querying and fetching. This PR addresses that by retrieving the value from doc values. There's a couple of limitations with this approach:

• values can no longer be fetched once doc_values are turned off for that field, as these are not stored in _source like ordinarily ingested fields
• fetching from doc values introduces format discrepancies as well sorting, same as docvalue_fields. That makes these calculated fields look different at fetch time compared to other fields

These limitations are not great, but I think that fixing the bug of not re-running the script at fetch time is valuable. Re-executing the script at fetch is unexpected: users may have indexed such scripted fields for better performance for instance, and reexecuting the script may lead to a different result, depending on the content of the script.

Closes #119939

[andreidan]

Thanks for working on this Luca!

Extra props for the PR description and all the tests ❤️

LGTM from a code changes perspective - pending other approvals.

[elasticsearchmachine]

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

[elasticsearchmachine]

Hi @javanna, I've created a changelog YAML for you.