Investigate incorrect CPU usage stats

During perf testing on a 48 core machine, one perf stat measuring the CPU usage via top was showing ~4000% (i.e. all cores running at ~85%)

However, the process_cpu_percent_utilization stat, was only ever showing about 800%

After looking at the code that calculates this stat a little (calculateProcessCpuPercentage), it's non-trivial, so we need to verify it's doing the right thing, and determine the reason for the huge difference above.