serverless mode: agent as prometheus exporter

There are two tickets in Kea (kea#279 (closed) and kea#280 (closed)) that requested having Prometheus exporter and Grafana template. We could address this request using stork agent. The agent would be working in exporter-only mode.

This should be reasonably simple to implement. There should be a command line switch (or some entry in the config file) that would prevent agent from opening grpc socket and only open Prometheus export socket.

(on a somewhat related note, there should be a way to do the opposite - only open grpc and not open Prometheus socket. That's to address scenario where user is not interested in Grafana).