Metric collection subsystem: watches the engine's metric registry, samples it on a cadence, and delivers snapshots to whatever sink the deployment has configured (an in-process listener, a periodic logger, an external exporter via the OTel subsystem). Interceptors hook into individual metric updates so a sink can react to events rather than poll.
The crate produces no metrics of its own; the engine's metric/ crate is the source. This subsystem only owns
the delivery path. New sinks plug in as listeners; new metric kinds belong in metric/.