Add JFR events for the garbage collector

[jiekang]

As title says. It would be nice to have events to cover configuration, pauses, heap and metaspace data.

[jiekang]

@christianhaeubl @christianwimmer

Hi! We'll be looking to add these. However there is one significant blocker at the moment, the location of code:

JFR is in com.oracle.svm.jfr
GC is in com.oracle.svm.core.genscavenge

How do you envision JFR events are added to core functionality, including more than the GC? Do we move the 'native' bits into com.oracle.svm.core.jfr? Or maybe expose some kind of listener API for core pieces similar to com.oracle.svm.core.thread.ThreadListenerSupport?

[christianwimmer]

@jiekang The GC projects (like genscavenge) should implement the JFR support directly without any further indirection. I think this can be achieved fairly easily by splitting the current com.oracle.svm.jfr project into a com.oracle.svm.core.jfr and com.oracle.svm.hosted.jfr part. The core part depends only on the svm.core project, and then genscavenge can depend on svm.core.jfr.

Looking at the source code (but without doing any refactoring myself), I think there are very few parts that need to be in svm.hosted.jfr. I see JfrFeature and JfrEventSubstitution have imports from svm.hosted, but nothing else.

[fernando-valdez]

Native JFR event support changes merged from this PR #3770