Add support for Sampling function (`traces_sampler`)

[gdubicki]

trafficstars

Hi,

The performance metrics of our apps are skewed by a lot of super-fast queries from our load balancer to the healthcheck endpoints.

We would appreciate implementing the Sampling function (traces_sampler), as documented here, with a mention:

Sampling functions (traces_sampler) are currently not supported by this platform.

This way we could implement checking the transaction path/URL and set the sample rate 0 for the ones that are to the healthcheck endpoints.

Please let us know if you are open to PRs with such a feature!