RFR: 8351187: Add JFR monitor notification event [v2]

[Aleksey Shipilev]

On Wed, 5 Mar 2025 06:51:36 GMT, David Holmes <dholmes at openjdk.org> wrote:

> Again a reasonable idea but note that notifications can occur much more frequently than waits. A wait only happens when a thread actually has to wait, whereas notifications tend to happen whenever a data structure is updated in a key way

On one hand, this looks like another reason to keep the event disabled by default. 

On the other hand, the way the event is currently implemented, it only fires when wait-set is not empty, plus or minus race conditions that would be resolved later in `INotify` after taking the internal lock. So there _should_ be a `wait` waiting for the overwhelming majority of eventful `notify`-es. Meaning, the number of `notify` events should be more or less tracking the number of `wait` events. It is likely the number of `notify` events are less than number of `wait` events even, if we assume most of the robust concurrent code uses `notifyAll()`.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/23901#issuecomment-2700789087