RFR: 8364282: ZGC: Improve ZPageAllocation JFR event sending

[Stefan Karlsson]

On Tue, 29 Jul 2025 13:51:16 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

> Hello,
> 
> This is a follow up to [JDK-8350441](https://bugs.openjdk.org/browse/JDK-8350441), improving on how the ZPageAllocation JFR event is sent. 
> 
> The starting time for the event is measured separately from creating a local event varaible (EventZPageAllocation). This means it is better to use the anonymous EventZPageAllocation::commit rather than creating an event and sending that event, which would record the starting time again, unnecessarily.
> 
> Additionally, if the event is not enabled (either specifically or if JFR is disabled), we should not waste time sampling statistics for the event.
> 
> Testing:
> * Oracle's tier1-4

Marked as reviewed by stefank (Reviewer).

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

PR Review: https://git.openjdk.org/jdk/pull/26534#pullrequestreview-3067716485