Add performance counter multiplexing

[double-fault]

Fixes #1813, #1896.

Lot of the code already existed but it was incomplete, like tracking of multiple leaders already existed but there was no extrapolation based off time running.

There were multiple groups in #1896 and the later groups always returned zero as the first group was pinned.

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[double-fault]

@dmah42 @LebedevRI (or whoever is the maintainer) ping for review. I think it's a pretty important bug, especially since there is no warning that there is no multiplexing, which leads to issues such as #1896 where I remember I had spent a lot of time on an unrelated program confused about why the counters were giving zero values.

If this patch is improper/there is no one to review it, it would be nice if a warning could be added somewhere regarding the same.

[dmah42]

I'm not an expert in the perf counter stuff but I'm going to take a look a bit later today

[dmah42]

i think this looks good. i've approved but maybe @LebedevRI wants to also weigh in.

[LebedevRI]

I'm sorry, i'm unfamiliar with this code. The change looks fairly simple.
Presumaby the author has tested this extensively and found no issues?

[double-fault]

@LebedevRI Yes the changes are fairly simple and I have tested to some extent - I have run it on programs with a lot of counters and the values seemed to make sense (such as the program I had described in #1896 ), but I have not tested it to check the accuracy of the values reported in some objective way. I am not aware of any easy method to do this, if there is maybe you could let me know and I can do it.

From what I remember, none of the unit tests check for the accuracy of the actual values either (as I said, I'm not sure if there is a proper way to do this).

[double-fault]

@LebedevRI ping

[LebedevRI]

@LebedevRI ping

I've already provided all the feedback that i have.