Follow up to https://github.com/elastic/apm-agent-dotnet/issues/906. With https://github.com/elastic/apm-agent-dotnet/pull/1020 we already mark sample rate on transactions and spans. The next step is to propagate it in the `tracestate`. Spec [here](https://github.com/elastic/apm/blob/master/specs/agents/tracing-sampling.md#propagation).
