Support Exemplars

[ktoso]

It's an experimental feature to corelate traces to metrics.

OpenMetrics introduces the ability for scrape targets to add exemplars to certain metrics. Exemplars are references to data outside of the MetricSet. A common use case are IDs of program traces.

Exemplar storage is implemented as a fixed size circular buffer that stores exemplars in memory for all series. Enabling this feature will enable the storage of exemplars scraped by Prometheus. The config file block storage/exemplars can be used to control the size of circular buffer by # of exemplars. An exemplar with just a traceID= uses roughly 100 bytes of memory via the in-memory exemplar storage. If the exemplar storage is enabled, we will also append the exemplars to WAL for local persistence (for WAL duration).

https://prometheus.io/docs/prometheus/latest/feature_flags/#exemplars-storage

Not sure entirely where these are attached when emitting metrics, probably need some extra metadata param or something