instrumentation for java.util.concurrent classes

Wrappers for `ThreadFactory` and `ExecutorService`.

`ThreadFactory` metrics:

* number of threads created.
* number of threads running.
* number of threads that completed.

`ExecutorService` metrics:

* number of tasks submitted.
* number of tasks running.
* number of tasks completed.
* timer for task time measurements.

Let me know what you think.