Add GHC.Stats.gcStatsEnabled

It would be nice if GHC.Stats exported a new, top-level constant, gcStatsEnabled :: Bool, that's set to True if the program was run with RTS -T or equivalent. In addition we could consider throwing an exception if getGCStats was called when gcStatsEnabled = False. This would let programs check if GC stats collection is enabled and still do something reasonable if they're not. This would be useful in the ekg package.

Open question: could the value of +RTS -T change at runtime? If so, perhaps we instead want getGCStatsEnabled :: IO Bool.

Trac metadata

Trac field	Value
Version	7.4.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
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CC	ezyang
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