Users can add @ExportMetric[Reader,Writer] to readers and writers that
they want to participate in the default exporter. There is also still an
@ActuatorMetricWriter that is used for the legacy (non-Java8) Gauge and
CounterServices.
The redis export and aggregate use case is a lot nicer with this
shared data between the two component types.
Also made MetricExportProperties itself a Trigger (so the default
delay etc. can be configured via spring.metrics.export.*).
This avoids a potential problems with ordering between Dropwizard and
normal repository configuration. A Dropwizard sample has been added to
verify the behaviour.
