[mdatagen] Add semconv reference for metrics in metadata schema

[ChrsMark]

Description

This PR adds support in mdatagen for defining SemConv reference for metrics.

Enhancing your metrics/signals in metadata.yaml like bellow:

metrics:
  system.cpu.time:
    enabled: true
    stability:
      level: beta
    description: Monotonic cumulative sum int metric enabled by default.
    unit: s
    sum:
      value_type: int
      monotonic: true
      aggregation_temporality: cumulative
    semantic_convention:
      semconv_ref: https://github.com/open-telemetry/semantic-conventions/blob/v1.37.0/docs/system/system-metrics.md#metric-systemcputime

Will provide the following generated docs:

system.cpu.time

Monotonic cumulative sum int metric enabled by default.

Unit	Metric Type	Value Type	Aggregation Temporality	Monotonic	Stability	Semantic Convention
s	Sum	Int	Cumulative	true	beta	system.cpu.time

Validation is added so as to ensure that provided SemConv urls match the top_level semconv version as well as the metric's name.

Link to tracking issue

Related to #13297 and #13910.

Testing

Tuned

Documentation

Tuned

[codecov]

Codecov Report

❌ Patch coverage is 86.08696% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.63%. Comparing base (5ec1466) to head (d1ca767).

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/metric.go	51.51%	11 Missing and 5 partials ⚠️

❌ Your patch check has failed because the patch coverage (86.08%) 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   #13920      +/-   ##
==========================================
- Coverage   91.67%   91.63%   -0.05%     
==========================================
  Files         652      652              
  Lines       42516    42626     +110     
==========================================
+ Hits        38977    39060      +83     
- Misses       2731     2750      +19     
- Partials      808      816       +8     

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[codeboten]

i like this change a lot. would be great to get this in