Add prometheus-emitter docs (#11618)

* Add prometheus-emitter docs

* Update docs/development/extensions.md

Co-authored-by: Charles Smith <techdocsmith@gmail.com>

Co-authored-by: Charles Smith <techdocsmith@gmail.com>
diff --git a/docs/development/extensions.md b/docs/development/extensions.md
index c2bccb0..49f61c8 100644
--- a/docs/development/extensions.md
+++ b/docs/development/extensions.md
@@ -94,6 +94,7 @@
 |druid-momentsketch|Support for approximate quantile queries using the [momentsketch](https://github.com/stanford-futuredata/momentsketch) library|[link](../development/extensions-contrib/momentsketch-quantiles.md)|
 |druid-tdigestsketch|Support for approximate sketch aggregators based on [T-Digest](https://github.com/tdunning/t-digest)|[link](../development/extensions-contrib/tdigestsketch-quantiles.md)|
 |gce-extensions|GCE Extensions|[link](../development/extensions-contrib/gce-extensions.md)|
+|prometheus-emitter|Exposes [Druid metrics](../operations/metrics.md) for Prometheus server collection (https://prometheus.io/)|[link](./extensions-contrib/prometheus.md)|
 
 ## Promoting community extensions to core extensions