message: refactor metadata to use get_or_insert_with
diff: @@ -123,19 +123,8 @@ impl Visitor for StructuralSchemaRewriter {
  if let Some(Schema::Object(variant_object)) =
  variant_obj.properties.values_mut().next()
  {
  - match variant_object {
  - SchemaObject {
  - metadata: Some(metadata),
  - ..
  - } => {
  - metadata.description = Some(description.to_string());
  - }
  - _ => {
  - let mut metadata = Metadata::default();
  + let metadata = variant_object.metadata.get_or_insert_with(|| Box::new(Metadata::default()));
  metadata.description = Some(description.to_string());
  - variant_object.metadata = Some(Box::new(metadata));
  - }
  - };
  }
  }
  }
  