GraphQL Stitching is losing "extensions" information from sub-schema response

[nick-stephen]

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Describe the bug

Stitching a remote schema loses any "extensions" information in the query response of a sub-schema

A GraphQL query response may have a top-level "extensions" field next to the "errors" and "data" fields:

https://spec.graphql.org/June2018/#sec-Response-Format

This "extensions" section can be used to provide auxilliary information such as profiling information or hints back to callers for performance optimization, or indeed anything else.

This information is lost when stitching in the remote graphql schema into a stitched global schema.

I would expect to be able to provide a merge function for the optional 'extensions' part of the response payload, with a default implementation which would naively merge the map contents.

[Warxcell]

I was able to track it down to this function, where extensions are lost

graphql-tools/packages/delegate/src/checkResultAndHandleErrors.ts

Line 48 in 1d0e1b9

export function mergeDataAndErrors(

but I'm not sure how to proceed.