Add exemplar support

[emschwartz]

This adds a Layer for tracing that exposes the Span fields to autometrics. It can then look for a field called trace_id and then automatically attach that as an exemplar for the metrics it generates.

TODO:

• In the tracing example, run Prometheus with the --enable-feature=exemplar-storage flag and actually scrape the metrics and exemplars

Resolves #26

[gagbo]

LGTM! That was a lot easier to implement than I thought it would :) I just added a few non-blocking nitpicks

[hatchan]

Already discussed out of bands, but I do feel like we should support the opentelemetry-otlp crate, which will be what most users will be using that are using the tracing crate. This crate will handle crossing the bridge from the tracing crate to the opentelemetry sdk. You would be able to use the following snippet to fetch the trace_id: Span::current().context().span().span_context().trace_id();

So in that sense I do agree with @gagbo that that maybe it would make sense to use a different name for the field as otherwise it would be confusing with the actual tracing.