feat: Adds exemplars

[isaacsanders]

This is a first draft of an implementation for exposing exemplars.

I used the following documentation to implement exemplars that consist of labels, a value, and a timestamp: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars

[isaacsanders]

I've also use the guidance discussed in this issue to determine how I would expose the option to the user: beam-telemetry/telemetry_metrics#96

[isaacsanders]

bumping on this.

[isaacsanders]

@bryannaegele Any thoughts on this?

[bryannaegele]

I looked at the docs you listed and looked at the prometheus docs. It seems that exemplars are experimental in prometheus and appears to be an experimental feature flag within a prometheus server. Is that accurate?

[weaversam8]

@bryannaegele it may be officially listed as "experimental" but major players in the ecosystem do support them. Grafana for example supports both displaying and querying exemplars as well as ingesting them thru their hosted Prometheus offering.

My team is considering using this library and would greatly benefit from exemplar support.