feat(metrics): add collector for sql.DBSats
This MR adds a Prometheus collector to expose sql.DBStats metrics.
Note: This implementation is heavily inspired by https://github.com/dlmiddlecote/sqlstats. I thought about embedding/wrapping this dependency, but given how simple the code is, I decided to replicate it here with a few changes:
- Functional options to configure the collector, aligned with the rest of the LabKit API;
- Custom metric names that conform with our style/conventions;
- Granular tests.
Related to #30.
Edited by João Pereira