Update 3.12.0 release notes

Author: michaelklishin

release-notes/3.12.0.md

@@ -89,6 +89,16 @@ RabbitMQ 3.12.0 will require **all** feature flags from the 3.11.x release serie
 
 If the feature flags are not enabled, RabbitMQ 3.12 and later nodes will refuse to start.
 
+### MQTT Client Metrics
+
+Some metrics displayed by the management UI are no longer available for MQTT clients.
+They are computed for AMQP 0-9-1 channels, which MQTT plugin no longer uses. They may be
+reintroduced in future releases when more protocol implementations (e.g. AMQP 1.0) are
+refactored the same way the MQTT plugin was for this release.
+
+Connection-level metrics such as ingress and egress data flows are still available,
+and so are the metrics of queues used by MQTT subscribers.
+
 ### Minimum Supported Erlang Version
 
 Starting with this release, RabbitMQ requires Erlang 25.0 or later versions. Nodes **will fail to start**