[mdatagen] Set metrics' default stability to development

[ChrsMark]

Description

Follow-up of #13756. Sets default metric's stability to development.

Link to tracking issue

Fixes #

Testing

Tuned

Documentation

~

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.37%. Comparing base (ba0b327) to head (f8fa649).

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/metric.go	50.00%	1 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (88.23%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13870   +/-   ##
=======================================
  Coverage   91.37%   91.37%           
=======================================
  Files         649      649           
  Lines       42653    42661    +8     
=======================================
+ Hits        38973    38981    +8     
  Misses       2854     2854           
  Partials      826      826           

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[codeboten]

I'd suggest alpha as per the documentation https://opentelemetry.io/docs/collector/internal-telemetry/#telemetry-maturity-levels

Alternatively we could add development to that doc, but there's no guarantees with alpha

[ChrsMark]

Moving this to draft since we proceeding with setting explicitly all metrics in contrib for now: open-telemetry/opentelemetry-collector-contrib#42809

Next step, if we agree on this, is to make madatagen to require the stability field.