code: func MetadataFromContext(ctx context.Context) (Metadata, error) {
  	value := ctx.Value(metadataKey)
  	if value != nil {
  		metadata, ok := value.(Metadata)
  		if ok {
  			return metadata, nil
  		}
  	}
  	return nil, errors.New("context contains no metadata")
  }
docs: // MetadataFromContext extracts Matadata from a given context's value.