Capture database connection pool metrics

[SylvainJuge]

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

Proposal

• use the OpenTelemetry semantic conventions for database metrics 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