Expose Akka's Executor stats

[jczuchnowski]

Akka's Dispatcher is a Java Executor. We should be able to expose its current state at least similarly to what Slick is doing.

[vpavkin]

It appears that Slick has a custom wrapper around the raw executor. And this wrapper is collecting and serving metrics.
I'm not sure the same thing is possible without a custom wrapper. And it's a lot to ask from the end user to put this custom executor in their app.
We can get some data using Thread.getAllStackTraces in a hacky way, but it also feels pretty bad.