chore: added documentation for prometheus

doc/stats.adoc

@@ -17,3 +17,15 @@ We currently measure:
 Under the hood, we are happy to rely on https://github.com/felixge/node-measured[measured] for all our metrics needs.
 
 To modify or simply access our stats in your plugin, simply `require('ep_etherpad-lite/stats')` which is a https://yaorg.github.io/node-measured/packages/measured-core/Collection.html[`measured.Collection`].
+
+
+=== Prometheus scraper
+
+Besides the non standard `/stats` endpoint, Etherpad also exposes a `/stats/prometheus` endpoint which is compatible with Prometheus scraping. It includes a lot more metrics than the standard `/stats` endpoint. It contains the following metrics:
+
+- ueberdb stats
+- gc
+- memory
+- event loop lag
+- v8
+- and more