Suppress warning when metrics PeriodicExecutor is stopped multiple times (e.g., during tests)

[tjquinno]

Resolves #3616

Change warning message to fine when PeriodicExecutor is shut down multiple times. This can happen if multiple @HelidonTests run in the same JVM.

This should not happen normally in production servers...the PeriodicExecutor stop is triggered when MetricsSupport receives notification of server shutdown, and there is typically at most one MetricsSupport instance in a server.

Also, while I was here, I changed synchronized methods to use a Semaphore instead.