Capture database connection pool metrics

Agent should be able to capture the database pool metrics in a generic way, hence allowing to create dashboards more easily.

This was reported quite a few times already and discussed recently [in our forum](https://discuss.elastic.co/t/spring-boot-jdbc-pool-metrics/332549)

### Proposal

- use the OpenTelemetry [semantic conventions for database metrics](https://github.com/open-telemetry/semantic-conventions/blob/main/specification/metrics/semantic_conventions/database-metrics.md) for metric names & labelling
- attempt to use our own metrics implementation as an attempt to minimize allocation
- cover only a few common database connection pool implementations as a 1st step
- tomcat dbcp
- hikaricp
- optionally cover common relational database drivers
- mysql / mariadb
- postgresql
- h2
- follow-up with dashboard creation with those metrics