Add PPROF_MUTEX_PROFILING_PERCENT env var details

[clayton-cornell]

PPROF_MUTEX_PROFILING_PERCENT enables the ability to retrieve data from the pprof mutex endpoint. It needs to be set to a whole number, for example 50 would profile 50% of the mutexes. 5 is likely a sufficient value. By default we sample 0.01 percent.

PPROF_BLOCK_PROFILING_RATE also allows tracking of mutexes for arcane reasons 1 tracks everything, 0 tracks nothing. Any value over 1 is treated as track anything over nanoseconds. By default we apply 10,000 for tracking.

Originally posted by @mattdurham in #1137 (comment)

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[clayton-cornell]

@mattdurham A question about the description provided for PPROF_MUTEX_PROFILING_PERCENT. The env var needs to be set to a while number (integer)? But... the default is 0.01 percent? Those two things don't add up for me. 0.01 isn't a whole number... should that have been 1 (meaning 1%)?