Implement Sampling Seed Propagation

[kahest]

Description

See https://github.com/getsentry/projects/issues/316

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.

Develop: Spec

See also

Sample implemeantion in PHP getsentry/sentry-php#1793.

[stefanosiano]

This should be shipped in 6 weeks by now!