cmd/mdatagen: introduce additional metadata for deprecated metrics

[Syedowais312]

This PR fixes how mdatagen handles deprecated metric fields in metadata.yaml.

The metadata loader now aligns correctly with the schema and test expectations,
ensuring deprecated fields are only set when explicitly defined.

What was changed?

• Updated metadata loading logic to correctly handle deprecated metric fields
• Ensured deprecated fields are only set when explicitly specified
• Aligned generated metadata with existing test expectations
• All existing and sample mdatagen tests pass after this change

---

Testing

• go test ./cmd/mdatagen/...
• Verified all sample metadata generators and internal tests pass
• All mdatagen tests pass.

---

Related: #14113

N/A (internal correctness and test alignment fix)

[codecov]

Codecov Report

❌ Patch coverage is 70.27027% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.81%. Comparing base (8ba3ae3) to head (e8dfbf9).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/metric.go	58.82%	5 Missing and 2 partials ⚠️
cmd/mdatagen/internal/metadata.go	33.33%	2 Missing and 2 partials ⚠️

❌ Your patch check has failed because the patch coverage (70.27%) 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   #14408      +/-   ##
==========================================
+ Coverage   91.80%   91.81%   +0.01%     
==========================================
  Files         677      677              
  Lines       42608    42700      +92     
==========================================
+ Hits        39116    39207      +91     
- Misses       2429     2434       +5     
+ Partials     1063     1059       -4     

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[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing Syedowais312:fix/mdatagen-deprecated-metrics (e8dfbf9) with main (55399d4)}

⚠️ 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

✅ 61 untouched benchmarks
⏩ 20 skipped benchmarks¹

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. ↩

[Syedowais312]

Note: generate-contrib failures are expected.

This PR enforces the metadata schema by disallowing deprecated under
telemetry.metrics. Several contrib components still use the old format
and need to migrate to stability: deprecated.

A follow-up issue will be opened in opentelemetry-collector-contrib.

[dmathieu]

This is starting to look nice 😄

[Syedowais312]

This is starting to look nice 😄

Thanks! Please let me know if any further changes are needed.

[dmathieu]

@Syedowais312 there is one open comment left 😺

[Syedowais312]

@Syedowais312 there is one open comment left 😺

@dmathieu thanks for the clarification. I’ve resolved the comment.

[ChrsMark]

LGTM

[ChrsMark]

Hmm, interesting that it was flattened already 🤔 .
We will handle the flattening in follow-ups in any case, but the levels here should be <development|alpha|beta|stable|deprecated> based on https://opentelemetry.io/docs/collector/internal-telemetry/#metrics.

[dmathieu]

IMHO, adding that should be its own PR though.

[dmathieu]

Ping @dmitryax for review, since you created the original issue.