Generic interface for metric exports. As you scale up metric collection you will often
need to buffer metric data locally and export it periodically (e.g. for aggregation
across a cluster), so this is the marker interface for those operations. The trigger of
an export operation might be periodic or even driven, but it remains outside the scope
of this interface. You might for instance create an instance of an Exporter and trigger
it using a @Scheduled annotation in a Spring ApplicationContext.