Add a section describing the metrics and labels produced by the libraries

Ideally, this would be a more user-friendly version of https://github.com/autometrics-dev/autometrics-shared/blob/main/SPEC.md#metrics. Then we can remove doc sections like https://docs.rs/autometrics/0.5.0/autometrics/index.html#generated-metrics and just link to a nice section of the docs website.