Feature request: add metric for isolated cores on cpu collector

[DavidVentura]

Host operating system: output of uname -a

Linux 5.11.0-40-generic #44-Ubuntu SMP Wed Oct 20 16:16:42 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

node_exporter version: output of node_exporter --version

$ ./node_exporter --version
node_exporter, version 1.3.1 (branch: master, revision: 7dbf35891570f9ce3bccb25a55176ea4923b35dd)
  build user:       david
  build date:       20211215-10:36:48
  go version:       go1.16.2
  platform:         linux/amd64

node_exporter command line flags

Mostly irrelevant but --collector.disable-defaults --collector.cpu

Are you running node_exporter in Docker?

No

What did you expect to see?

# HELP node_cpu_isolated Whether each core is isolated, information from /sys/devices/system/cpu/isolated.
# TYPE node_cpu_isolated gauge
node_cpu_isolated{cpu="0"} 0
node_cpu_isolated{cpu="1"} 1
node_cpu_isolated{cpu="2"} 1
node_cpu_isolated{cpu="3"} 1

What did you see instead?

I have a (crude) working version for this, which I'd like to PR soon if this feature is acceptable.