[metaspace] provide size histogram for jcmd VM.metaspace

In order to analyze Metaspace workloads (in preparation for JDK-8221173) and find optimal chunk allocation strategies, it is useful to know the size distribution of metaspace allocations, especially per loader space type.

Add an option to jcmd VM.metaspace to print a size histogram for both class- and non-class space.

Collecting those information should be controlled by switch and by default off.