Eventlog should include delimiters showing when the process writes to the .eventlog file

Quoting a comment I just added at https://ghc.haskell.org/trac/ghc/wiki/EventLog#Limitations :

Event logging can produce "observer effects". In particular, programs can pause while writing to the .eventlog file. These display in Threadscope as periods of 100% CPU use with no obvious cause. This effect is exacerbated by heavy use of debug tools which expand the eventlog file, such as Trace.traceMarkerIO. (This effect was at least seen on OS X. In an extreme case, a program doing nothing but writing 200,000 trace markers, which took 120 ms to run, showed a single 10ms pause in the middle to write them to the .eventlog file. These periods were shown to be due to writing the .eventlog file by using dtruss -a, which shows long periods of write_nocancel syscalls, writing to the .eventlog file opened earlier with open_nocancel.)

This behavior caused a few hours of investigation for us. It would make it much more obvious what is happening if this behavior were logged in the eventlog. Specifically, I would like for the eventlog to include a "starting writing eventlog file" event at the start of each of these periods, and a "stopped" event at the end of each period.

Trac metadata

Trac field	Value
Version	7.10.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
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CC	simonmar
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