Add support for persistence metrics

[nscuro]

This PR adds metrics support for both DataNucleus and HikariCP, providing insights into how the persistence layer of Alpine is doing.

While DataNucleus provides a more high-level view of things, the HikariCP metrics will hopefully allow users to better tune their pool size.

Note: The DataNucleus statistics are not thread safe until datanucleus-core 6.0.2 (see https://github.com/datanucleus/datanucleus-core/pull/478) and may yield weird numbers as a consequence. They'll start to emit reliable numbers as soon as we update to 6.0.2.

Here's an example of how this may look in a Grafana dashboard: