fix: Another attempt at fixing this weird issue

[JuanPedroGHM]

An attempt at #152

• Changed monitoring subprocess to spawn instead of fork to have a clearer separation between main and subprocess resources.
• Explicit delete and close of backends on subprocess, to avoid dangling file descriptors from the rapl backend.
• More logging.

[codecov]

Codecov Report

Attention: Patch coverage is 54.81481% with 61 lines in your changes missing coverage. Please review.

Project coverage is 62.67%. Comparing base (0a04ca0) to head (70ce6ae).

Files with missing lines	Patch %	Lines
perun/monitoring/subprocess.py	4.87%	39 Missing ⚠️
perun/monitoring/monitor.py	75.71%	17 Missing ⚠️
perun/backend/nvml.py	20.00%	4 Missing ⚠️
perun/backend/rocmsmi.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
- Coverage   63.13%   62.67%   -0.46%     
==========================================
  Files          31       32       +1     
  Lines        2517     2575      +58     
==========================================
+ Hits         1589     1614      +25     
- Misses        928      961      +33     

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[JuanPedroGHM]

This, did not work.
Next attempt:

• Only one subprocesses, for all the rounds. Should remove some overhead, might be more stable. Need to write logic for a reusable subprocesses.