Generic Couchbase cluster discovery framework

CMOS-34 will add a process for "discovering" Couchbase clusters from Prometheus targets. The framework is generic enough that this could be used for other sources (k8s, for example), however currently the implementation is very Prom-specific (e.g. CLI flags just for Prometheus). It'd be good if we had a more generic way of setting up discovery.

Setting priority=minor as Prometheus will be enough in many cases.