Add support for GraalVM Native Image using java.util.logging

[loicmathieu]

logstash-gelf version: 1.13.0

When using the library with an application build as a native image via GraalVM, it didn't works. No stacktraces are shown but no logs are issued.

To reproduce the issue, use the library inside a GraalVM native image and you will see that no log events are sent.

I chased down the issue to JulLogEvent.getThreadName(LogRecord). This method uses the ThreadMXBean to get the name of the thread from it's ID, but SubstratVM (the VM that runs the native image compiled by GraalVM) didn't implements JMX so this could not work.

I have a working solution that I can contribute via a PR to replace the usage of the ThreadMXBean by Thread.getAllStackTraces().keySet() however I don't know if it has some performance implication (as it uses a cache maybe not important).

[mp911de]

Happy to include changes that make logstash-gelf work in GraalVM native image runtimes.

This project is in maintenance mode meaning that we fix bugs and keep dependencies up to date but there's no one that could drive major improvements unless we get contributions in the form of pull requests.

[loicmathieu]

OK I'll prepare a PR soone

[mp911de]

Fixed via #214.