It would be good if the JMX API included standard types for metrics and statistics. Compare the statistics defined by J2EE 1.4:
http://java.sun.com/j2ee/1.4/docs/api/javax/management/j2ee/statistics/package-summary.html

The J2EE statistics types suffer from the problem that they are interfaces with no standard implementation classes. This means that they cannot reliably be serialized between a server and a client unless the client deploys implementation-specific classes appropriate for the particular server it is talking to. This can probably be avoided in the context of JMX by using a java.lang.reflect.InvocationHandler that is backed by a javax.management.openmbean.CompositeData.