[mdatagen] Add semconv reference for metrics in metadata schema

[ChrsMark]

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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 https://github.com/open-telemetry/opentelemetry-collector/issues/13910 and https://github.com/open-telemetry/opentelemetry-collector/issues/13297.

Testing

Tuned

Documentation

Tuned