remove reflective access from the executors instrumentation, fixes #641

[ivantopo]

We are finally getting rid of reflective access on JDK classes! This should remove the annoying warning (that turned into an error by default in Java 17:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by kamon.instrumentation.executor.ExecutorInstrumentation$ (file:/home/xxxx/.ivy2/cache/io.kamon/kamon-bundle_2.12/jars/kamon-bundle_2.12-2.4.8.jar) to field java.util.concurrent.Executors$DelegatedExecutorService.e
WARNING: Please consider reporting this to the maintainers of kamon.instrumentation.executor.ExecutorInstrumentation$
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

A side effect of this is that we no longer can instrument certain executors returned by java.util.concurrent.Executors), including the ones from:

• newSingleThreadExecutor
• newSingleThreadScheduledExecutor
• unconfigurableExecutorService
• unconfigurableScheduledExecutorService

That shouldn't be a big issue, I hope.

[dpsoft]

I think that the only metric that could be important is the queue-size in the newSingle*, but I agree that shouldn't be an issue(big).