feat(processor/openstack): Add monitor agent for servers metadata

[gfieni]

This PR adds an OpenStack pre-processor monitor agent that periodically syncs server metadata from the OpenStack API into a shared cache, mirroring the Kubernetes pre-processor metadata flow.

Detailed changes:

• Add OpenStackMonitorAgent to refresh server metadata in the background.
• Add polling-interval configuration flag to setup the monitor agent polling interval.
• Improve libvirt cgroup parsing for cgroup v1 and v2 paths.
• Add unit tests for the openstack pre-processor module.

[codecov]

Codecov Report

❌ Patch coverage is 71.62162% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.12%. Comparing base (24d3d33) to head (fc3b0e6).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
.../powerapi/processor/pre/openstack/monitor_agent.py	72.91%	13 Missing ⚠️
src/powerapi/processor/pre/openstack/actor.py	55.55%	4 Missing ⚠️
src/powerapi/processor/pre/openstack/handlers.py	40.00%	3 Missing ⚠️
src/powerapi/cli/common_cli_parsing_manager.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #825      +/-   ##
==========================================
+ Coverage   71.18%   72.12%   +0.93%     
==========================================
  Files          89       90       +1     
  Lines        2957     2999      +42     
==========================================
+ Hits         2105     2163      +58     
+ Misses        852      836      -16     

Files with missing lines	Coverage Δ
src/powerapi/cli/generator.py	88.99% <100.00%> (+0.05%)	⬆️
src/powerapi/processor/pre/openstack/_utils.py	100.00% <100.00%> (+66.66%)	⬆️
.../processor/pre/openstack/metadata_cache_manager.py	100.00% <100.00%> (+44.44%)	⬆️
src/powerapi/cli/common_cli_parsing_manager.py	0.00% <0.00%> (ø)
src/powerapi/processor/pre/openstack/handlers.py	72.41% <40.00%> (+37.11%)	⬆️
src/powerapi/processor/pre/openstack/actor.py	66.66% <55.55%> (-1.76%)	⬇️
.../powerapi/processor/pre/openstack/monitor_agent.py	72.91% <72.91%> (ø)

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