test: add coverage for gen-metric-doc tool

[vprashar2929]

This commit adds the tests for validating the gen-metric-doc tool.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.70%. Comparing base (c3b4722) to head (f70409e).
Report is 2 commits behind head on reboot.

Additional details and impacted files

@@            Coverage Diff             @@
##           reboot    #2137      +/-   ##
==========================================
+ Coverage   86.22%   91.70%   +5.48%     
==========================================
  Files          34       34              
  Lines        2991     2991              
==========================================
+ Hits         2579     2743     +164     
+ Misses        369      194     -175     
- Partials       43       54      +11     

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