perf: only instrument knex if necessary

[watson]

trafficstars

If captureSpanStackTraces is disabled, there's no reason to capture the intermediate knex stack trace which is only used to improve the span stack traces of pg and mysql spans.

[watson]

@Qard I haven't written any tests for this as I wasn't sure exactly how I could test this in a reasonable way. If you have any suggestions that aren't too complicated let me know 😃

[Qard]

You could just check for existence of the unwrap function on Knex.Client.

[trentm]

Obsoleted by https://github.com/elastic/apm-agent-nodejs/pull/2879