Making performance trace sample rate configurable

[tobias-wilfert]

Addressing: #1515 still some todo's that need to be tackled but opened to invite feedback.

TODO's:

• Find a way to write some tests for this (or none if deemed not necessary)
• Discuss if the documentation is sufficient
• Discuss if the variable naming is intuitive and what to do with the now 'out-dated' comment

[loewenheim]

Looks good to me! I think tests aren't necessary for this, but it could use a changelog entry.