[DOCS] Document Metricbeat limitation related to some metrics

[lucabelluccini]

The missing field is when the process is not around long enough to be included in two periods of metric collection. Because it is the first time that the process exists in the period it does not include the field.

In order to calculate percentages, the process needs to appear at least twice to calculate a performance delta. We can expect that the first time a process is reported from the metricset that it does not have for example a system.process.cpu.total.norm.pct value. The second time it is report it then will have a system.process.cpu.total.norm.pct value. In this case process is too short lived then it will only ever be reported once having no value in system.process.cpu.total.norm.pct.

Please note that versions earlier than 8 were reporting the field with 0, when we shouldn't have been reporting anything. Versions on 8.x are not populating the field.

Why do we need 2 values?
For example, the CPU usage in percent is calculated based on a time delta using the raw tick value (usually representing a millisecond) that indicates the total amount of CPU time a process has used. For example, if we collect the tick value for a process in 5 second intervals, and and the difference between the first tick value and the second is 1000 ticks, then we know the process has used 1 second of time in a 5 second interval, or 1/5, or 20%, of CPU time. In order to do this, we need three numbers: The first count of total cpu time, the second count, and the time between the counts. Hence why we need two values.

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!

[lucabelluccini]

bump