Send some DSC information in envelope header

[lcian]

For correct extrapolation of span metrics we need some dynamic sampling information to be included in (transaction) envelope headers (trace envelope header aka DSC).
Otherwise, the backend will assume sample rate = 1 and extrapolate incorrectly.
In theory these values should be relied upon only for dynamic sampling, but we also use them for metrics extrapolation, leading to this requirement of sending them from the SDK even if it doesn't currently support Dynamic Sampling.

Specifically, we need to send out the following envelope headers for all transactions:

• trace_id
• public_key
• sample_rate
• sampled

[linear]

RUST-88 Send DSC information in envelope header