Update 4.1.0 release notes

Author: ansd

release-notes/4.1.0.md

@@ -1,5 +1,23 @@
 ## RabbitMQ 4.1.0
 
+## Highlights
+
+### AMQP 1.0 Filter Expressions
+
+[PR #12415](https://github.com/rabbitmq/rabbitmq-server/pull/12415) implements `properties` and `appliation-properties` filters of [AMQP Filter Expressions Version 1.0 Working Draft 09](https://groups.oasis-open.org/higherlogic/ws/public/document?document_id=66227) when consuming from a stream via AMQP 1.0.
+String prefix and suffix matching is also supported.
+
+This feature:
+* adds the ability to RabbitMQ to have multiple concurrent clients each consuming only a subset of messages while maintaining message order, and
+* reduces network traffic between RabbitMQ and clients by only dispatching those messages that the clients are actually interested in.
+
+### Prometheus histogram for message sizes
+
+[PR #12342](https://github.com/rabbitmq/rabbitmq-server/pull/12342) exposes a Prometheus histogram for message sizes received by RabbitMQ.
+
+This feature allows operators to gain insights into the message sizes being published to RabbitMQ, such as average message size, number of messages per pre-defined bucket (which can both be computed accurately), and percentiles (which will be approximated).
+Each metric is labelled by protocol (AMQP 1.0, AMQP 0.9.1, MQTT 5.0, MQTT 3.1.1, and MQTT 3.1).
+
 ## Potential incompatibilities
 
-* The default MQTT [Maximum Packet Size](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901086) changed from 256 MiB to 16 MiB. This default can be overriden by [configuring](https://www.rabbitmq.com/docs/configure#config-file) `mqtt.max_packet_size_authenticated`. Note that this value must not be greater than `max_message_size` (which also defaults to 16 MiB).
+* The default MQTT [Maximum Packet Size](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901086) changed from 256 MiB to 16 MiB. This default can be overridden by [configuring](https://www.rabbitmq.com/docs/configure#config-file) `mqtt.max_packet_size_authenticated`. Note that this value must not be greater than `max_message_size` (which also defaults to 16 MiB).