prometheus
feat(prometheus-client-derive): initial implemenation
Summary
This PR implements features proposed by #140, supersedes #218 (which doesn't makes new progress in the past several months).
This PR adds and publishes (to crates.io) a new crate prometheus-client-derive, and re-exports its macros in crate prometheus-client, so the user should use prometheus-client instead of prometheus-client-derive. This re-export pattern is also used by serde.
The new crate only has one macros Registrant derive the trait that allows structs to be registered in a registry.
Futures of derive-encode
This PR doesn't touch it, but I think we should merge it into prometheus-client-derive eventually as the first one's functionalities is a subset of the second one.
Since this is a breaking change, this is not the call should be made by outside contributor, even if we can do breaking change when releasing 0.24.0 based on the Rust semantic version.
Issues
close #140, supersede #218
