multi-field collapse support

[lifangming]

Please use the following questions as a guideline to help me answer
your issue/question without further inquiry. Thank you.

Which version of Elastic are you using?

[ ] elastic.v7 (for Elasticsearch 7.x)

Please describe the expected behavior

multi-field collapse query support

Please describe the actual behavior

it seems NewCollapseBuilder only support one field

Any steps to reproduce the behavior?

[olivere]

I suppose you're referring to this feature?

[milesich]

Is this issue referring to have multiple inner hits inside collapse? Because I was just looking for it.

[olivere]

@milesich I haven't looked into it, but I assume that's it, yes.