The current release of `tracing` includes the predecessor of valuable (https://docs.rs/tracing-core/latest/tracing_core/span/struct...). Valuable extracts this functionality and improves on it, but hasn't quite made its way back into tracing yet. I expect that it will be included in upcoming releases (I know eliza has been poking me to release valuable and get it in tracing, I probably should get on that!)