Finalization has provided several ways to observe the overall level of finalization activity in the system (e.g. the number of objects yet to be finalized, or pending finalization). These include:

* MemoryMXBean.getObjectPendingFinalizationCount()
* jcmd GC.finalizer_info
* jmap -finalizerinfo
* jdk.FinalizerStatistics JFR event

With the deprecation of finalization, Cleaner becomes the primary mechanism for releasing resources when an object becomes unreachable.

We should provide a similar level of observability for Cleaner.