Merge pull request #934 from prometheus/update-exemplar-docs

Add documentation for how to export exemplars

Author: Julien Pivotto

README.md

@@ -252,6 +252,20 @@ h = Histogram('request_latency_seconds', 'Description of histogram')
 h.observe(4.7, {'trace_id': 'abc123'})
 ```
 
+Exemplars are only rendered in the OpenMetrics exposition format. If using the
+HTTP server or apps in this library, content negotiation can be used to specify
+OpenMetrics (which is done by default in Prometheus). Otherwise it will be
+necessary to use `generate_latest` from
+`prometheus_client.openmetrics.exposition` to view exemplars.
+
+To view exemplars in Prometheus it is also necessary to enable the the
+exemplar-storage feature flag:
+```
+--enable-feature=exemplar-storage
+```
+Additional information is available in [the Prometheus
+documentation](https://prometheus.io/docs/prometheus/latest/feature_flags/#exemplars-storage).
+
 ### Disabling `_created` metrics
 
 By default counters, histograms, and summaries export an additional series