statsmanager: add rust type and ffi for managing common metrics

[jkarneges]

This adds a statsmanager Rust module along with a CommonMetrics type and FFI for managing the kinds of metrics that the C++ StatsManager class would need if it were to rely on Rust code for managing its Prometheus metrics.

For now this code is unused, but the plan is to change StatsManager to call CommonMetrics::update with the values from its combinedReport field. The values in combinedReport are cumulative, so update computes deltas to increment by.

Note: normally, we should declare Prometheus metrics within the specific components that use them rather than in core (e.g. connmgr's metrics are declared in the connmgr module), however since StatsManager is already in core it is easier to start out by putting the metrics it needs alongside it, hence naming them "common" metrics. Later on we can consider teasing them out somehow into separate components.