Skip grouping-related stack trace enhancers for background threads

[Swatinem]

AFAIK, only the main thread / exception stack trace contributes anything to grouping. However the stack trace enhancers are being run for all the threads, and we have seen some extreme outliers with up to 400+ stack traces per event.

We should skip running grouping-related enhancers for all the stack traces that are not relevant for grouping.
Or phrased the other way around: We should only run grouping-related enhancers for the stack traces that end up matter for grouping.