Emit query duration metric

The extension currently doesn't emit any SQL-specific k6 metrics. A basic one that would be useful for tracking DB performance would be `query_duration`. It should be possible to apply thresholds and tags per query, as mentioned in #22.

While this could be implemented in JS [as shown in this example](https://github.com/grafana/xk6-sql/issues/22#issuecomment-1195323702), it's a very imprecise method.

## Suggested solution

Golang's `database/sql` doesn't seem to expose any such statistics, but there's an example [here](https://medium.com/@ducmeit/integrate-hook-operations-for-tracking-time-finish-for-a-sqls-query-at-golang-267471e325a2) that wraps `driver.Driver` to calculate the value. It's essentially doing what the above JS example does, but in Go, so it's not great, but it's an improvement nonetheless.