@fabio-venturi I am not familiar with Prometheus, but I asked the AI on the FusionAuth site and it came back with.
Database_primary_pool_Usage is a Prometheus metric exposed by FusionAuth which reports how much of the primary database connection pool is currently in use. It lets you see whether your HikariCP pool is close to exhaustion and is useful for capacity and health monitoring. [Monitor Prometheus]
In the Prometheus UI you can graph it by entering Database_primary_pool_Usage in the expression box and executing the query. [Monitor Prometheus]
It said it based the answer on the page you found, but I don't know enough to say for certain. Does this make sense to you?