[4.x] - Remove non-virtual executor support

[dalexandrov]

Resolves #7272
Removes non-virtual executor support

[dalexandrov]

After some investigations I found out that java.util.concurrent.ThreadPerTaskExecutor is package private, so it cannot be accessed without reflection.
So there are two options:

• keep everything as is. Currently this infrastructure is done only for one metric threadCount which is got out of package private public long java.util.concurrent.ThreadPerTaskExecutor.threadCount(). This infrastructure allows accessing other potential "hidden" metrics, accessible via reflection.
• remove entire method invocation infrastructure and not publish this metric. This is done in this PR.

@tomas-langer and @tjquinno, please share your thoughts.

[tjquinno]

Generally, I have mixed feelings about the value of keeping this feature. The argument can be made that, in 4.x, exposing this metric is less important than in earlier releases with Helidon's use of virtual threads.

TBH, I do not have very strong feelings either way about keeping the metric (and therefore having to continue using reflection which we like to avoid) vs. removing the metric. If there are effective actions an operations person can take after monitoring this metric, then that does argue for keeping it. If not, then not!

[tjquinno]

If we are going to keep this feature (and the addition of the type checking before casting is good), then we might as well use instanceof pattern matching:

if (executorService instance ThreadPoolExecutor tpe) {
    registerMetrics(tpe, index);
}

I know it's a small stylistic point and functionally no different from doing explicit casting (ThreadPoolExecutor), but it is more concise and less prone to typos.

[dalexandrov]

Applied! Thank you!

[tjquinno]

IF we want to keep this feature I'm OK with the changes in the PR.

The question is whether we want to keep the metric. As I said, I could be convinced either way.

[tomas-langer]

@tjquinno please decide yourself, then either approve or request changes ;)
Thanks