Add traces_sample_rate behind an ENV var

What does this MR do and why?

This will add the option to enable traces_sample_rate, that will enable Sentry performance monitoring

Currently will only work on new-sentry

For now we just set traces_sample_rate instead of a more complex traces_sampler, to see if Sentry performance monitoring works as expected, and is something we'd like to use in the future

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #390232 (closed)