[Dynamic Sampling] Sampling Seed Propagation

[lcian]

To increase the chance of capturing complete traces when users return a new sample rate tracesSampler in backend services, we propagate the random value used by the SDK for computing the sampling decision instead of creating a new random value in every service. Therefore, across a trace every SDK uses the same random value.

Spec

As the Rust SDK is a backend SDK and we currently don't capture any traces for outgoing network requests, we only need to conform to the spec when it comes to the following points:

• read the incoming sample_rand or create one
• make a sampling decision based on the sample_rand
• send the sample_rand as part of the DSC in envelopes
• update traces_sampler to pass the parent_sample_rate

A prerequisite to implement this feature is to support Dynamic Sampling via creating, handling and sending the Dynamic Sampling Context, which is tracked here: #741