test(sdk): fix AWS Trainium test

[dmitryduev]

Description

• tests/unit_tests/test_system_metrics_trainium.py::test_trainium was causing trouble on the CI apparently bc I wasn't shutting down neuron_monitor_mock properly.
• added more guardrails around the Trainium monitor internals.

Testing

• Locally
• CI
• Real trn1 instance: https://wandb.ai/dimaduev/trainium/runs/yf6dkb0l?workspace=user-dimaduev

Checklist

• Ensure PR title compliance with the conventional commits standards

[codecov]

Codecov Report

Merging #4753 (cb370fb) into main (d17fc52) will decrease coverage by 0.04%.
The diff coverage is 7.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4753      +/-   ##
==========================================
- Coverage   83.86%   83.81%   -0.05%     
==========================================
  Files         286      286              
  Lines       36025    36028       +3     
==========================================
- Hits        30213    30198      -15     
- Misses       5812     5830      +18     

Flag	Coverage Δ
functest	56.91% <7.14%> (+<0.01%)	⬆️
unittest	74.02% <7.14%> (-0.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wandb/sdk/internal/system/assets/trainium.py	76.47% <7.14%> (-2.58%)	⬇️
wandb/sdk/internal/artifacts.py	83.48% <0.00%> (-4.59%)	⬇️
wandb/sdk/internal/context.py	97.91% <0.00%> (-2.09%)	⬇️
wandb/sdk/internal/internal_api.py	88.83% <0.00%> (-1.37%)	⬇️
wandb/sdk/lib/printer.py	89.24% <0.00%> (-0.64%)	⬇️
wandb/sdk/wandb_init.py	86.52% <0.00%> (-0.55%)	⬇️
wandb/sdk/wandb_setup.py	88.38% <0.00%> (-0.51%)	⬇️
wandb/sdk/wandb_run.py	90.78% <0.00%> (-0.06%)	⬇️
wandb/sdk/lib/git.py	77.71% <0.00%> (ø)
wandb/util.py	87.68% <0.00%> (+0.41%)	⬆️
... and 3 more