Metrics

Initial stab at metrics, more to come later.
 - number of http/net connections & connections by remote address
 - number of http requests (client & server) and requests by uri (only for server)
 - http request latency and rate (client & server)
 - net/http bytes read/written. For http we only record the content/body. So this excludes headers and such.
 - number of event bus & handlers by address
- number of verticles & by verticle type (verticle.getClass().getName()).
It shouldn't use a weak reference as key, but WeakHashMap as we want to avoid any memory leaks if the response does not complete normally. I have on my todo to pass in a response in case there is an exception so we can remove error responses.