Add server-side sample rate to ingested spans

[jan-auer]

Span extraction places the client sample rate into the span's measurements for extrapolation in the event platform. For organizations with dynamic sampling, the server-side sample rate must be added in addition to that.

• Add an additional field for the server-side sample rate
• Consider to add a field for the combined sample rate
• Consider a dedicated field in the protocol outside of measurements

NB: The sample rate has to be used in the EAP spans consumer (here)