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 91.86%. Comparing base (8d9186a) to head (f551993).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14424   +/-   ##
=======================================
  Coverage   91.86%   91.86%           
=======================================
  Files         677      677           
  Lines       42777    42759   -18     
=======================================
- Hits        39295    39281   -14     
+ Misses       2427     2424    -3     
+ Partials     1055     1054    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

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

[dmathieu]

@dmitryax @ChrsMark This is ready for review.
Doing the contrib fixes, I have doubts if this isn't too much work for folks using mdatagen though (kinda like the components renames).
See the size of open-telemetry/opentelemetry-collector-contrib#45903

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will improve performance by 28.99%

_{Comparing dmathieu:mdatagen-flatten-stability (7288e7e) with main (42b573e)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

⚡ 1 improved benchmark
✅ 60 untouched benchmarks
⏩ 20 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	HTTP/1.1,_shared_client_(like_load_balancer)	1.4 ms	1.1 ms	+28.99%

Footnotes

1. 20 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[ChrsMark]

LGTM overall.

I agree the amount of changes is enormous but it's not quite user facing so I'd fine doing those to address the initial suggestion.

[dmitryax]

LGTM