Exemplar support

[emschwartz]

Prometheus and OpenTelemetry support attaching exemplars to metrics. It would be useful to provide support for this out of the box with autometrics.

Ideally, we would integrate with whatever popular libraries are used for to add tracing in each language so that we could automatically attach the trace_id to the metrics.

The biggest problem I foresee with this is that exemplars are not well-supported across many of the OpenTelemetry and Prometheus client libraries. We will either need to submit PRs to add support for exemplars upstream, wait until they are added, or support this feature only when autometrics is being used in combination with a metrics library that supports exemplars.

• [x] Spec https://github.com/autometrics-dev/autometrics-shared/pull/25
• [x] Rust https://github.com/autometrics-dev/autometrics-rs/issues/26 (only using the prometheus-client backend)
• [x] Python https://github.com/autometrics-dev/autometrics-py/issues/41 (only using the prometheus-client package)
• [ ] TS https://github.com/autometrics-dev/autometrics-ts/issues/57
• [x] Go https://github.com/autometrics-dev/autometrics-go/issues/44 (only using the prometheus client)
• [ ] C#
• [ ] Java

[emschwartz]

Since exemplar support is optional and in a number of cases blocked on upstream support for it in the Prometheus/OpenTelemetry client libraries, I am going to close this issue for now.