Mdatagen: flatten the metric stability level

[dmathieu]

This flattens the metrics stability level, to better match the weaver schema.
Closes #14113

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.78%. Comparing base (cac5574) to head (5ae5e95).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14424      +/-   ##
==========================================
+ Coverage   91.77%   91.78%   +0.01%     
==========================================
  Files         677      677              
  Lines       42430    42421       -9     
==========================================
- Hits        38940    38938       -2     
+ Misses       2428     2424       -4     
+ Partials     1062     1059       -3     

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[dmathieu]

To avoid losing this information, we probably want to wait until #14408 has shipped.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.