feat: Performance Monitoring traces sampler callback

[Hugo-C]

Following discussion in #73 , this should allow the usage of traces_sampler as exposed by Sentry.
I kept the fairing method for compatibility.
I need to check the fairing_init_with_specific_traces_sampler that seems to be failing for some reasons.

[Hugo-C]

For the failing test, if fairing_init is duplicated it'll sometime fail. The problem is not with fairing_init_with_specific_traces_sampler then. So I guess we can't have Hub, Rocket or something else in parallel :(
I'll add a lock so these two tests run in sequence.
EDIT: using Hub::current() instead of Hub::main() seems to be working fine.

[intgr]

Thanks! Some initial thoughts.

[Hugo-C]

@intgr Thanks for the review. Please take another look when you have some time.

[intgr]

Sorry, will try to get to it tomorrow. I had a brief look some days ago and it seemed pretty close to finished.

Don't hesitate to ping me again if I drop the ball 😅

[intgr]

Thanks for your patience. :) I found some more suggestions.

[Hugo-C]

Thanks for your patience. :) I found some more suggestions.

I had some busy weeks. The PR should be all good now, I can also squash the commits if needed.

[intgr]

Looks all good to me, thanks!

[intgr]

I can also squash the commits if needed.

Nah, I use 'squash and merge'

[intgr]

Should be released now as rocket-sentry v0.18.0