Bring back `sparse_vector` mapping

[benwtrent]

Description

We have rank_features and its useful as is. However, when comparing with new retrieval techniques, the name rank_features isn't expected.

Users expect sparse_vector as a partner with dense_vector.

So, we should bring back sparse_vector. It may initially be a simple copy of rank_features.

See history: #48368, #48781

[elasticsearchmachine]

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

[kderusso]

Unassigning this for now, as first priority once someone is free.

[carlosdelest]

This is the work that I'll start doing for this task:

• Change SparseVectorFieldMapper implementation, so it:
  • is no longer deprecated
  • checks for 7.x and versions less than 8.11 and warn / error appropriately
  • extends from RankFeaturesFieldMapper, and just changes the CONTENT_TYPE. We could make a complete copy, but I think we can save some duplicated code this way (and we might revisit this decision later)
• Create sparse_vector docs
• Change ELSER docs to refer to sparse_vector field type instead of rank_features. We can add compatibility notes to ensure users understand they can use ELSER with rank_features and sparse_vector interchangeably - but that sparse_vector is the field to be used onwards to refer to it.

Anything I'm missing?

Some questions I would like your opinion on:

• Should we address explicitly that sparse_vector is equivalent to rank_features? Or at least mention they are compatible? (I'd say yes)
• Should we bring back script vector functions that operate on sparse_vectors? (I'd say yes as they can be used on scripts and feel natural to provide a simplified way to calculate cosine similarity for example)
• Should we add back sparse_vector feature usage? (I'd say yes so we can compare adoption of this field type vs rank_features)

[benwtrent]

extends from RankFeaturesFieldMapper, and just changes the CONTENT_TYPE. We could make a complete copy, but I think we can save some duplicated code this way (and we might revisit this decision later)

Please, just copy paste. Especially since rankfeaturesfieldmapper is in a module and sparsevector will not be.

Change ELSER docs to refer to sparse_vector field type instead of rank_features.

We should't do that until the ELSER queries are updated and such. Don't do anything related to ELSER in the sparse_vector work. It should be done in follow up PRs to ensure we focus on the correct things.

Should we address explicitly that sparse_vector is equivalent to rank_features?

IMO, no, we should not. They do the same thing for now but they might diverge and sparse_vectors may support things that rank_features may not and vice versa.

Should we bring back script vector functions that operate on sparse_vectors?

Maybe in a separate PR. This is not as important as getting the field mapping back and switching ELSER.

Should we add back sparse_vector feature usage? (I'd say yes so we can compare adoption of this field type vs rank_features)

Yes, we should

[carlosdelest]

Thanks for the feedback, @benwtrent !

I created this draft PR. Can you please take a look to ensure it's in line with your thoughts?