Fix servo HTTP metrics.

The HttpClientListener and HttpServerListener were listening for HttpClient/ServerMetricEvents instead of Client/ServerMetricEvents.
This causes ClassCastExceptions while receiving Client/ServerMetricEvents.

The test did not catch this as the listener in the test was a TCP listener attached to an HttpClient. Since all Http*MetricEvents can be converted to *MetricEvents and not the other way around, the tests were happy.
Optimized imports
Fix for servo metrics (contd)

Now added the servo listeners to the examples which also covers the basic sanity of these event types.
Merge pull request #163 from NiteshKant/master

Fix servo HTTP metrics.