XD-855: Change metrics assertions in integration tests to use smart Thread.sleep

* Applied the "fixture" pattern to metrics related sinks (same as other sinks and sources)
 * Only did the MetricsTest class, marked the other constructs as deprecated for the moment
* Used Hamcrest matchers to assert the state of those sink fixtures (e.g. exists(), or hasValue(X))
* Also added the use of an eventually() construct (that may be used elsewhere)