[receiver/hostmetrics] Use gopsutil total seconds to calculate utilization

[rubenruizdegauna]

Description:
system.cpu.utilization delta time is calculated using timestamps read in each scrape. These timestamp reads are not directly related to the cpu times reads so the used delta time to calculate utilization is not aligned with the delta time from times values. This makes system.cpu.utilization inacurate.

As cpu utilization relies on cpu times, the delta time should be calculated from the cpu times value (gopsutil already provides a Total() function that returns the total number of seconds). This way we can avoid timestamp read and persistence between scrapes simplifying it.

Now:

utilization.user = cpu.user / (time diff from system time)
utilization.idle = cpu.idle / (time diff from system time)
...
Proposal:

utilization.user = cpu.user / (elapsed seconds from times)
utilization.idle = cpu.idle / (elapsed seconds from times)
...

Link to tracking Issue:
#8856

Testing:
Tests have been refactored.

Documentation:
No additional documentation has been added.

[dmitryax]

I don't think that utilization.user = cpu.user / (cpu total seconds) is what utilization is supposed to be. If we do this, we are getting overall utilization right from the boot time. Once significant amount of time passes this value will come to some amount that won't be changing anymore.

For example:

Let's assume utilization.user at t1 is 50000000/100000000 = 0.5. Then CPU was idle next 10 seconds from t1=100000000 to t2=100000010, so utilization.user at t2 is still very close 0.5 50000000/100000010 = 0.49999995.

So if we do this change the metrics becomes useless.

[dmitryax]

Oh, after looking at the code, seems like utilization.user = cpu.user / (cpu total seconds) is just a misleading description. We still have utilization.user = cpu.user / (time diff) just calculated differently :)

[rubenruizdegauna]

Oh, after looking at the code, seems like utilization.user = cpu.user / (cpu total seconds) is just a misleading description. We still have utilization.user = cpu.user / (time diff) just calculated differently :)

my bad 😅 I have updated the description and the variable name in 0e0c90e

[rubenruizdegauna]

Hi @dmitryax do you think this can be merged to main? Is there anything else missing from my side? Thanks!