Add processes status stats in system input plugin

[titilambert]

It returns the number of processus grouped by their status (running/sleeping/zombie/...)

[titilambert]

This add metrics asked here #235

[sparrc]

this looks OK but I'm a little worried about the processing required for it, this is going to have to read every single process file in /proc/ and parse out all of the information, and then return the processes....

it likely would be more efficient to use ps, as well as working on both linux and bsd/darwin

[sparrc]

from what I can tell, ps axo stat= will work on bsd/darwin/linux systems

[titilambert]

@sparrc I understand, but I just did what collectd does ( but maybe this is not a right way :D see here: https://github.com/collectd/collectd/blob/master/src/processes.c#L1826 )
If you prefer the ps way, I will change it ;)

[sparrc]

fair enough....but I don't think this is limited to linux, is it? it looks like gopsutil supports the status info for freebsd, darwin, etc.

[titilambert]

Maybe there is some other status for those OSes ?
And windows compatibility ?

[sparrc]

For all unix systems these should be it, I have no idea about Windows, but generally I've been trying to encourage people to use windows perf counters for system metrics