Address a long standing \todo item that suggested to implement a read-only interface for the Request::metadata() accessor and deflect to the internal implementation for the read-write accessor used by pipeline handlers. Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com> Acked-by: Paul Elder <paul.elder@ideasonboard.com> Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>