Provide RTS statistics in GHC.Stats, for example number of blocked threads etc
Motivation
The current GHC.Stats mainly exposes GC details, and memory usage details. But it doesn't expose anything about number of running/blocked... threads? It can be insightful when troubleshooting production issues to see for example that the number of blocked threads (for example blocked on some MVar / TVar) have suddenly increased.
Now, I have no idea what of this kind of thread/MVar/TVar/.. stats is feasible to collect/expose in the runtime, but the more the merrier.
Proposal
Basically adding new stats fields about threads or synchronization primitives (STM retry counts, whatever).
Note that these are preferred in a Prometheus-friendly incremental counter format, like many of the memory-related stats already are.