cpu: add workaround for counter resets related to % Processor Utility metric

[jkroepke]

Fixes #1299

Problem

The % Processor Utility performance counter hold 2 values: Amount of busy time (V), amount of CPU tick (D).

To calculate the CPU percentage, the follow formula is needed: (V1 - V0) / (D1 - D0). In prometheus, it can calculates via rate(V[1m]) / rate(D[1m]).

The CPU tick counter is an unsigned int32, with overflows approx. each 2 hours and the rollover happens on 4294967295 (2^32).

Program, that fetching the raw values, can handle the counter reset, because interger substractor works on overflows, too. (go ref: go.dev/play/p/57GXzfT-m5G)

For example:

C0 = ((2^32)-10)
C1 = 10
C1 - C0 = 20

The problem here is that all Prometheus values are float64. By converting the values, the bit based mechanic is lost.

This image explain how Prometheus handle the overflow which results into gaps using the rate function

Solution

The solution will introduce a dedicated counter which do some interger calcuation on each scrape. and add the value to a float64 counter