metrics: relay correlated metrics to Prometheus

[mythi]

Collected raw metrics are already relayed to Prometheus. Rebalancing will probably produce a number of high-level, domain-specfic metrics, more suitable for decision making within cri-resmgr. These metrics should be exported to Prometheus as well.

[klihub]

This PoC branch implements relaying all collected raw metrics to Prometheus.

[msivosuo]

Part of #114