Update 3.9.9 release notes

(cherry picked from commit 6182505)

Author: michaelklishin

release-notes/3.9.9.md

@@ -43,6 +43,12 @@ Contributors are encouraged to update them together with their changes. This hel
 
 ### Prometheus Plugin
 
+#### Enhancements
+
+ * Metric rendering efficiency improvements (mostly latency)
+
+   GitHub issue: [#3587](https://github.com/rabbitmq/rabbitmq-server/pull/3587)
+
 #### Bug Fixes
 
  * TLS-enabled Prometheus endpoint listener port was not correctly stored internally