Make sampled trace work correctly with nested standard instrumentation

[JonasKunz]

Given a call hierarchy where root() calls nested(). If only root() is instrumented and has stack-trace sampling enabled, nested() will appear as a sampled span in the resulting trace.

However, if nested() also is configured with start-span: true, it will appear twice in the trace: One time due to standard instrumentation and one tiem due to the stack trace sampling performed by root().

The goal of this issue is to seamlessly integrate stack trace sampling with nested instrumentations.