Add explain capabilities to parent/child queries

[martijnvg]

Currently the has_child, has_parent and top_children queries don't support score explanation, which results in not implemented yet... messages in the explain response if one of these queries is used.

[mgreene]

I would love to see this. It's currently very difficult to troubleshoot slow queries within ES and any insight into parent/child query execution would be very helpful and much appreciated!

[soeren-helbig]

+1

reconstruction of scoring with child/parent could be hard. would love to see it too.

[speedarius]

+1 - this would be very nice to have

[gmenegatti]

+1

[mmahalwy]

+1

[Gasol]

👍

[martijnvg]

This was implemented via #6107.