GC sync time is not available from getRTSStats as a cumulative counter
## Summary
That makes it impossible to get an accurate view of time spent in sync phase when using prometheus-style sampling. This was something I wish we had in production recently.
It's available for the most recent GC: https://hackage-content.haskell.org/package/base-4.22.0.0/docs/GHC-Stats.html#t:GCDetails
But there is no companion to the `gc_elapsed_ns` counter.
**EDIT**: per the comment in `GHC.Stats` and CLC discussion thread, I will add this new field to `GHC.Internal.Stats` (and by extension `GHC.Stats.Experimental`) only. `GHC.Stats` will keep the same API.
I'm working on a PR for this.
## Environment
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issue