Add a way to uninstrument JVM metrics

If I call `metrics.jvm.core/instrument-jvm` during startup, there is no corresponding `metrics.jvm.core/uninstrument-jvm` to call during shutdown. This causes problems for me during tests that check startup and shutdown behavior; if I call `instrument-jvm` twice I get an error "A metric named jvm.attribute.vendor already exists".

I suppose a less desirable alternative that would still address my use-case would be to make instrumentation idempotent. A clean shutdown would be my preference.