Record sampling weight to transactions/spans and propagate

[vigneshshanmugam]

See elastic/apm#307.

We need to record a sampling weight on transactions and spans, propagating the value through tracestate header for downstream services.

• Document fixing CORS issue

[hmdhk]

We should consider this a breaking change since adding tracestate header would break existing applications due to CORS.

We should also document how to fix the CORS issue the same way we have done for traceparent.