Expose the number of "in process" (i.e. unacknowledged messages given to the client)
via the JMX interfaces.

e.g. change
org.jboss.mq.server.jmx.Queue to have
public int getInProcessCount()
{
return jmsQueue.getInProcessCount();
}

org.jboss.mq.server.JMSQueue to have
public int getInProcessCount()
{
return basicQueue.getInProcessCount();
}

org.jboss.mq.server.BasicQueue to have
public int getInProcessCount()
{
return unacknowledgeMessages.size();
}