Find a better place for counter utility classes after JEP387

Followup to JEP387:

JEP387 comes with a number of new utility classes, shyly hidden away inside the metaspace source tree. Some of those could be brushed up and made general purpose classes instead.

One example are the counter classes (see: http://cr.openjdk.java.net/~stuefe/jep387/review/2020-08-11/webrev-core/webrev/src/hotspot/share/memory/metaspace/counter.hpp.html )