[Linux] `cpu_percent` does not work within `oneshot` context manager as expected

Summary

OS: Ubuntu 20.04
Architecture: x86 64bit
Psutil version: 5.8.0
Python version: 3.8.10
Type: doc, core (?)

Description

Not sure if this is a bug, but this behaviour is not documented at all and has left me scratching my head for couple of hours. Basically, cpu_percent method does not work as described with non-zero interval argument when called in oneshot context. My test scripts:

With `oneshot`

import psutil
p = psutil.Process(41264)
with p.oneshot():
    print(p.cpu_percent(interval=0.5))

prints 0.0

With `oneshot` and explicit `sleep`

import psutil
import time
p = psutil.Process(41264)
with p.oneshot():
    p.cpu_percent()
    time.sleep(0.5)
    print(p.cpu_percent())

prints 0.0

Without context manager

import psutil
p = psutil.Process(41264)
print(p.cpu_percent(interval=0.5))

Prints 16.0 or other non-zero value, as expected.

Suggestions

If this is by design, I think it should be mentioned in the documentation under oneshot and/or cpu_percent descriptions. Currently documentation for the context manager lists cpu_percent among methods that gain a speedup from it and even shows a code that doesn't seem to work:

with p.oneshot():
    p.cpu_percent()  # return cached value

giampaolo / psutil