GraphQL response metadata/annotation

[jychen7]

Is your feature request related to a problem? Please describe.
In REST api, the query response of /v1/load contains whether it is slow query and serving from external storage.
https://github.com/cube-js/cube.js/blob/c4f51f457dbebec60f4848b951d2604de2512631/packages/cubejs-api-gateway/src/gateway.ts#L1534-L1536

However, in GraphQL api, seems this information is missing. This information is quite useful to monitor how many queries hitting the pre-aggregation/rollup.

Describe the solution you'd like
I understand GraphQL response body is align with the request. However, is it possible to include this in response header?

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

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