Document extension as value for stability in metadata.yaml (open-tele…

…metry#23426)

Currently, this is used in all extensions (e.g.
[httpforwarder](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/extension/httpforwarder/metadata.yaml#L6)),
but this is not documented in `metadata-schema.yaml`.

cmd/mdatagen/metadata-schema.yaml

@@ -7,12 +7,12 @@ status:
  class: <receiver|processor|exporter|connector|extension>
  # Required: The stability of the component - See https://github.com/open-telemetry/opentelemetry-collector#stability-levels
  stability:
- development: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics>]
- alpha: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics>]
- beta: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics>]
- stable: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics>]
- deprecated: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics>]
- unmaintained: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics>]
+ development: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics|extension>]
+ alpha: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics|extension>]
+ beta: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics|extension>]
+ stable: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics|extension>]
+ deprecated: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics|extension>]
+ unmaintained: [<metrics|traces|logs|traces_to_metrics|metrics_to_metrics|logs_to_metrics|extension>]
  # Optional: The distributions that this component is bundled with (For example core or contrib). See statusdata.go for a list of common distros.
  distributions: [string]
  # Optional: A list of warnings that should be brought to the attention of users looking to use this component