katana: expose db metrics

[kariy]

expose database metrics from the metrics server.

possible path to implement this:

• create a trait for db metrics (the trait shouldn't be specific to on-disk database stats because we have in-memory database as well)
• update the prometheus_exporter::serve() function to accept another argument for db metrics.
• extract db metrics everytime the prometheus endpoint is called.
• flatten the building process of node components so shared items can be easily passed around during the build process (ie have a single function to build everything?)