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()).
This is inefficient. HttpClientResponseImpl already contains a reference to HttpClientRequestImpl so no need to look it up in a Map. This means you only have to maintain one Map of HttpClientRequest-> TimedContext. (And I don't see why it has to be weak)