While I was developing [foyer](https://github.com/foyer-rs/foyer), which is a hybrid in-memory and disk cache project in Rust. Some users asked for metrics system support for both the Prometheus ecosystem and OpenTelemetry ecosystem. After investigating and trying out some libs, I decided to develop a new lib that is lightweight, support both Prometheus and OpenTelemety, and even different version of `prometheus`, `prometheus-client` and `opentelemetry` crates for various users.
