feat(report): Allow metadata to be sanitized and flattened

[gfieni]

PR overview:

• Add the sanitize and flattening methods for the metadata dict of the reports ;
• Update the PowerReport to sanitize/flatten its metadata dict for InfluxDB or Prometheus outputs ;
• Rework of the report tags generation method ;
• Improve unit tests for the Prometheus and InfluxDB power report serialization.

Fixes: #408

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.02%. Comparing base (d21f096) to head (015ebf1).
Report is 56 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   67.92%   68.02%   +0.10%     
==========================================
  Files          71       71              
  Lines        2977     2987      +10     
==========================================
+ Hits         2022     2032      +10     
  Misses        955      955              

Files with missing lines	Coverage Δ
src/powerapi/report/power_report.py	76.47% <100.00%> (-0.28%)	⬇️
src/powerapi/report/report.py	93.65% <100.00%> (+1.34%)	⬆️