Support CUE_STATS_FILE with the SDK

[verdverm]

Is your feature request related to a problem? Please describe.

CUE_STATS_FILE can be set be end users of the CLI to get helpful info about how their code runs in the evaluator. Users of the SDK cannot make use of this (accurately) because some of the stats API requires access to internal packages

cue/cmd/cue/cmd/root.go

Line 118 in 901604a

adt.ResetStats()

Describe the solution you'd like

CUE_STATS_FILE to work via the SDK

Describe alternatives you've considered

It doesn't seem possible to access stats directly because much of it requires access to the adt package

Additional context

hof user asked why this doesn't work with hof

[mvdan]

I've brought up the same to Marcel in the past, this would indeed be nice to have. We just have to be careful to clearly document that the structure of the stats is unstable and may change with future CUE versions. We could make the struct fully opaque to hide the fields from the user entirely, or use a map[string]any, although that also makes using the stats directly in Go rather annoying. I'd be fine with exposing a plain struct with exported fields as long as the documentation is clear.