Kibana: [Upgrade Assistant] Add support for chained multi-fields deprecation

[alisonelizabeth]

In 7.x, it was possible to define a multi-field within a multi-field. This feature was deprecated in 7.3 and will no longer be supported in 8.0. This issue is to consider how to support this change in Upgrade Assistant.

Related resources:

• Breaking changes in 8.0 doc
• https://github.com/elastic/elasticsearch/issues/41267

Note: It's expected that few users will have chained multi-fields defined, as it was an unintentional feature in ES.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[cjcenizal]

@alisonelizabeth @sebelga Does the mappings editor in 7.x support multi-fields in multi-fields?

[alisonelizabeth]

@cjcenizal I did a quick test yesterday. The UI does not allow you to create a multi-field within a multi-field. If you use the "Load JSON" functionality with one, there is no error, but the nested multi-field is stripped out. I did not test yet how the UI behaves if the template is first created via Console.

[cjcenizal]

Thanks @alisonelizabeth! You read my mind with your second point. We should make sure that loading and saving an existing index template that contains multifields-in-multfields (MIM) doesn't opaquely destroy anything. If it does then I would consider it a similar bug to the opaque types-removal bug we fixed. I think it's OK to disallow adding and working with MIM via the UI but we shouldn't opaquely destroy them.

[sebelga]

Yes, this is a similar bug than the automatic types removal. Is it as severe? It looks like it, anyone that upgrades from a minor 7.x or 6.8.6 to 7.6 and edit its template through the UI, will have its nested multi-fields removed.

I think it's OK to disallow adding and working with MIM via the UI but we shouldn't opaquely destroy them.

I think if the API allows it we should allow it in 7.x with a deprecation warning whenever the user creates a nested multi-fields.

[LeeDr]

I don't think we need the v7.6.2 label on this issue, do we? We're probably not going to make any changes around this in a patch release.

[alisonelizabeth]

@LeeDr I removed the label. Thanks for pointing that out!

/cc @sebelga I had originally opened this issue to track any changes needed to UA. I think it ended up leading to a discussion around the mappings editor, but I changed the issue title back to it's original intent 😄