Add support for exemplars to `prometheus_remote_write` sink

[jszwedko]

Requested by user shipping data to Grafana Cloud.

This appears to be an extension to the promotheus protocol to send labels that have high cardinality, for example trace ids, to be able to link together logs, metrics, and traces.

Note that this does use some unreleased features of Prometheus / Grafana so we should make this behavior opt-in unless it is compatible with existing versions.

References:

• https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars
• https://grafana.com/blog/2020/11/09/trace-discovery-in-grafana-tempo-using-prometheus-exemplars-loki-2.0-queries-and-more/