Quick documentation of the exemplars feature on the disabled features (#8655)

page.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

docs/disabled_features.md

@@ -38,3 +38,11 @@ More details can be found [here](querying/basics.md#offset-modifier).
 
 The remote write receiver allows Prometheus to accept remote write requests from other Prometheus servers. More details can be found [here](storage.md#overview).
 
+## Exemplars Storage
+
+`--enable-feature=exemplar-storage`
+
+[OpenMetrics](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars) 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 flag `storage.exemplars.exemplars-limit` can be used to control the size of circular buffer by # of exemplars. An exemplar with just a `traceID=<jaeger-trace-id>` uses roughly 100 bytes of memory via the in-memory exemplar storage.
+