Change span.set_data to span.set_attribute

Change start_transactions in the tests to start_spans

Stringify db.params, db.paramstyle, db.cursor

❗ Change how adding query source works.
Previously, we first let the DB span finish and then looked at its start and end timestamps to figure out if the query was slow enough to attach query data.

With the switch to OTel, we can no longer put stuff on a finished span, meaning the slow query span still has to be alive when adding data to it. So instead of looking at the end timestamp of a finished span, we keep the span alive and instead look at current time at the point when the query is finished.

This means the DB span will be longer than the actual query (since it now includes adding query data).