[Fields API] _type is not returned in the response

[majagrubic]

When switching Discover to use the Fields API, _type field is no longer returned in the fields response. Moreover, when explicitly requesting a list of fields (which includes _type), an exception is thrown:
type: "unsupported_operation_exception", reason: "Cannot fetch values for internal field [_type]."

Our understanding was that this would be supported until 8.0.0. Removing it in a minor could be considered a breaking change for Discover.

[elasticmachine]

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

[jtibshirani]

Thanks for raising this. For now maybe fields could ignore the _type field it does for all other metadata fields. We meant to ignore _type, but there's a bug in how we detect metadata fields -- I think we miss _type since it was removed in 8.x.

We have a dedicated issue on how to return metadata fields generally: #63569. It'd be great to get your feedback there if you have thoughts.

[jtibshirani]

Here's a summary from digging in further:

• On the 7.x branch, and for 7.x indices on 'master' we correctly ignore the _type field.
• On 'master' with 8.x indices, we throw an error on _type. Instead we should behave as if the field didn't exist, where we don't error but return no values.

I opened #68413 to fix this oversight on 'master' and will close this out once it's merged. If you need a change to how metadata fields like _type are handled in general, let's discuss on #63569.

Update: that PR was closed in favor of #68564.