Exemplar support

[emschwartz]

Not entirely sure what the support is like upstream: open-telemetry/opentelemetry-go#559

[gagbo]

It seems the blocking issue for autometrics is this issue open-telemetry/opentelemetry-go#3163

As long as that is not fixed I don't think there's much we can do. Exemplars as a datastructure attached to datapoints exist in the SDK codebase but I found no way to add Exemplars to counters in the OpenTelemetry codebase. All it can do is add "attributes" that end up being labels in the Prometheus export

The prometheus client does have support for adding exemplars it seems, but it looks like we need to specify what we want as exemplars, since both key and value are mostly free-form fields. I'm not sure what's the good way to try to extract the traceID/spanID in a "framework agnostic" way in Go, it'll take some research I think

[gagbo]

This is done for Prometheus, still waiting on something for opentelemetry, but we can probably just use tracing