Export metrics using manually configured scheduling rather than using @EnableScheduling

[wilkinsona]

Automatically enabling scheduling when actuator's on the class path has surprised a few people (#8040, #7307). It can be avoid it by setting spring.metrics.export.enabled to false but then you either lose metrics exporting or you having to configure it all manually. It'd be nice if metrics could be exported without manual configuration and without enabling scheduling for the whole app.

[wilkinsona]

This has been superseded by the planned move to Micrometer-based metrics (#9970)