RFR: 8358318: JFR: Tighten up PlatformTracer initialization [v2]

[Erik Gahlin]

On Mon, 2 Jun 2025 17:28:32 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Started as SonarCloud complaint that MethodSetting.apply initializes a static field. But I think there is a larger problem here, which I completely missed during [JDK-8352738](https://bugs.openjdk.org/browse/JDK-8352738) review: the `PlatformTracer` initialization can technically run several times, AFAICS. This PR tightens up the initialization. It also allows us to call `ensureInitialized` from other places, if we ever find a need.
>> 
>> 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 incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Unsynchronized fix
>  - Merge branch 'master' into JDK-8358318-jfr-platform-tracer-init
>  - Fix

Marked as reviewed by egahlin (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/25584#pullrequestreview-2889374075