improve GC allocation rate measurement

[brharrington]

In #1154 JFR support was added. This could be an opportunity to improve the behavior for the GC allocation rate meter. Currently it is based on the notification emitter events and we can only detect the amount of allocations when one of these GC events arise. This can mean that it takes a while to detect them and it appears to the user as if there are periodic massive spikes in allocations when in reality it is a small rate over time and the spike is just when the event was received. Example:

The actual behavior is a few hundred MB/s and it takes around 15m to fill up young gen and get a GC event. With JFR ideally we can improve this to better represent the actual behavior and reduce user confusion.