Remove metrics when shutting down InstrumentedQueuedThreadPool

[wakingrufus]

remove metrics from MeterRegistry when shutting down InstrumentedQueuedThreadPool
Something like:

    @Override
    protected void doStop() throws Exception {
       Set<Tag> tagSet = new HashSet<>();
        tags.forEach(tagSet::add);
        Stream.of("jetty.threads.config.min",
                        "jetty.threads.config.max",
                        "jetty.threads.busy",
                        "jetty.threads.jobs",
                        "jetty.threads.current",
                        "jetty.threads.idle")
                .flatMap(name -> meterRegistry.find(name).meters().stream())
                .map(Meter::getId)
                .filter(id -> new HashSet<>(id.getTags()).containsAll(tagSet))
                .forEach(meterRegistry::remove);
        super.doStop();
    }

Rationale
Clean up resources when not needed anymore

Additional context
The dropwizard metrics jetty InstrumentedQueuedThreadPool does something similar, and it seems like a good idea to do here as well.

I am happy to open a PR for this with tests if it is agreed that this should exist.

Thanks

[shakuzen]

It sounds like it may make sense, but I'd like to understand more. In most cases, I would expect stopping the threadpool to happen on shutdown of the Server which would also correspond to shutting down the JVM. In that case, I don't think it matters much if we remove the meters. Is this proposal for cases where Jetty Servers will be started and stopped multiple times while the JVM is running?

[wakingrufus]

Yes. Another place where it may be nice is with running multiple integration tests, spinning up and shutting down Jetty multiple times for different tests, but all using the same globalRegistry. This would provide better isolation for such tests.

[shakuzen]

We offer a clear method for cleaning up the registered meters on a global registry. Though we generally advise against using a global registry as much as possible to avoid having the issue in the first place. Nonetheless, it wouldn't be a bad idea for us to clean up as you suggest. Please do send us a pull request.

[wakingrufus]

Ok I have opened a PR for this. please take a look and let me know if there is anything I can improve.

Thanks again!