Run 'explain' for slow database queries

(Originally opened by @macnibblet as https://github.com/elastic/apm-agent-go/issues/520)

**Is your feature request related to a problem? Please describe.**
A feature I really like from Newrelic that I would love to see is transaction tracking of slow queries that exceed a certain threshold.

**Describe the solution you'd like**
If the duration for a query exceeds a configurable interval, then the hook integrations should generate a explain query and store it with the span.

This would also require integration with the APM server / UI.