Expose metrics for taskPool and taskQueue in XnioWorker

We are using Undertow and hence XnioWorker embedded in our executable war projects.

We are also using Dropwizard Metrics (http://metrics.dropwizard.io).

Our goal is to expose key metrics from Undertow to our monitoring platform. Specifically we want to track the thread usage level (active threads / total threads) and the worker queue length.

This is not currently possible because on XnioWorker the taskPool fiels is private and has no accessors and getWorkerQueueSize() method is protected (and it's not clear how one could provide a sub-class of this method).

If this is something of interest, it is possibly something I could make a PR for.