Metrics SPI should not be exposed measured instances

[vietj]

Motivation : currently the Metrics SPI expose the measured instances to the SPI implementation when at creation of the corresponding metrics instance. This instance is actually not really used by the implementations (Vertx instance was used by micrometer metrics instance to create an HTTP server but in practice it can start its own embedded instance as there is no much gain to create a new one). One issue providing the Vertx instance is that the provided instance was a racy publication of the Vertx instance complicated to solve (because the MetricsFactory needs to be called early for creating the pool metrics). The metrics event bus initialised callback is also not used in practice (it used to be by hawkular implementation), only by micrometer metrics as a signal to initialise the backend which can be replaced by a simple initialisation in the micrometer metrics factory.

Change : do not provide measured instances when calling the SPI for creating a metrics implementation. Callback methods have also been renamed to contain the type of the metrics created (i.e createMetrics -> createEventBusMetrics, etc...). The event bus initialised callback is removed as well.