RFR: 8337199: Add jcmd Thread.vthread_scheduler and Thread.vthread_pollers diagnostic commands [v2]

[Kevin Walls]

On Thu, 28 Nov 2024 15:54:54 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Adds `jcmd <pid> Thread.vthread_scheduler` to print the virtual thread scheduler and `jcmd <pid> Thread.vthread_pollers` to print the I/O pollers that support virtual threads doing blocking network I/O operations.
>> 
>> This is a subset of the diagnostics that we've had in the loom repo for a long time. @larry-cable proposed a PR recently ([pull/22121](https://github.com/openjdk/jdk/pull/22121)) to bring a version of same into main line but it was based on an older proposal. This new PR supplants that effort.
>> 
>> The jtreg failure handler is updated to execute Thread.vthread_scheduler, useful capture when a test fails or times out.
>
> Alan Bateman has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve regex for matching poller String representation

Marked as reviewed by kevinw (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/22414#pullrequestreview-2468508549