Native API for JFR event producers

[Nitsan Wakart]

Hi,
Picking up a conversation from the OpenJDK committers workshop, the
following feature was requested/suggested:
"Make the means of producing JFR events available to native agents"

The motivation for this request is to serve use cases where the events
published come from data sources which are naturally native, e.g.:
- Memory allocator fragmentation stats (`malloc_stats`)
- `perf_events`/BPF/dtrace/other profiling
- Native library counters

This would require something like:
- Settling on some extension to JVMTI to accommodate the API. Including
`JfrEvent`.
- Declaring/reserving event types statically.
- Recording lifecycle hooks to notify the event sources of
start/stop/configuration events.

It would make sense for the above to mirror the Java API as much as
possible for consistency.
In addition it would be good if the API allowed to describe native threads,
method calls, and stacks. This is to support the native profiling use cases.

Thanks everyone who participated in the discussion ( Eric, Ron, Alan, Mario
and others), I hope I capture the sentiment from our conversation
correctly, please chip in so we can flesh this out :-)

Regards,
Nitsan