Add meter for ForkJoinPool#getDelayedTaskCount

[shakuzen]

This new public method was added in Java 25. See https://github.com/openjdk/jdk/pull/23702/files#diff-e398beb49cd8d3e6c2f3a8ca8eee97172c57d7f88f3ccd8a3c704632cab32f5fR3924-R3936.

Returns an estimate of the number of delayed (including periodic) tasks scheduled in this pool that are not yet ready to submit for execution. The returned value is inaccurate while delayed tasks are being processed.

We already monitor getQueuedTaskCount, but as now noted:

The returned count does not include scheduled tasks that are not yet ready to execute, which are reported separately by method getDelayedTaskCount.

The corresponding documentation should be updated.

[shakuzen]

We'd want to run a test for this with a Java 25 JVM, which isn't available for the CircleCI convenience image we currently use (cimg/openjdk). We could rely on the Gradle auto-provisioning feature to download a JRE to run the test task that requires Java 25, but it'd be ideal to not need to download a runtime during the build.