MessageQueue is used in multiple ways, for both incoming and outgoing messages, but this PR focuses on NatsConnectionWriter which is used for outgoing messages.

NatsConnectionWriter is used by NatsConnection to publish messages. (The concept of Producer is a Kafka concept and is more analogous to your specific application component that publishes messages. NATS itself has no concept of a Producer, and for that matter using broker is confusing as well as that is also a Kafka concept.)

The MessageQueue's
length is a count of items [messages] in the queue. sizeInBytes is the number of bytes calculated from the serialized form of all the messages in the queue.

The change to NatsConnectionWriter exposes the MessageQueue's length and sizeInBytes with two new internal (packaged scoped) messages that specifically expose the Outgoing MessageQueue's information

long outgoingPendingMessageCount() {
    return outgoing.length();
}

long outgoingPendingBytes() {
    return outgoing.sizeInBytes();
}

The change to the Connection interface and the NatsConnection implementation exposes publicly the length and sizeInBytes from the NatsConnectionWriter.

public long outgoingPendingMessageCount() {
    return writer.outgoingPendingMessageCount();
}

public long outgoingPendingBytes() {
    return writer.outgoingPendingBytes();
}