Refine Metrics API/SPI

The current metrics API mixes in pure metrics aspects with reporting ones. The factories should not require the reporting duration unit nor the rate unit nor display names. These should be chosen when using reporters, whatever the metrics collection backend.

Let's use Stream<Metric> instead of Iterable in MetricsFactory::registered().

@TimingCapture concern should name timers with some "Structure path" by default, like we do in jmx support, and allow to name the underlying timer with an annotation parameter.