RFR: 8351187: Add JFR monitor notification event [v4]

[Markus Grönlund]

On Thu, 6 Mar 2025 13:17:42 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> We have `JavaMonitorWait` event, but no symmetric `JavaMonitorNotify` event. Notifications are important/interesting to track as well, for example to correlate the delay between notification and eventual wake up.
>> 
>> Providing this event would also replace one of of the RT counters that are going away in [JDK-8348829](https://bugs.openjdk.org/browse/JDK-8348829).
>> 
>> This counter is disabled by default to keep any potential impact low. We can consider flipping it to enabled by default later.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `jdk_jfr`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:
> 
>  - Only emit event when notification happened
>  - Merge branch 'master' into JDK-8351187-jfr-monitor-notify
>  - Rewrite test to RecordingStream
>  - Drop threshold to 0ms
>  - Merge branch 'master' into JDK-8351187-jfr-monitor-notify
>  - Disable by default
>  - Fix

Unsure if this event type carries enough weight. The JavaMonitorEvent already has a notified field:

    <Field type="Thread" name="notifier" label="Notifier Thread" description="Notifying Thread" />
    
Generally, any event that cannot be enabled by default needs good motivations.

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

PR Comment: https://git.openjdk.org/jdk/pull/23901#issuecomment-2704778898