Handle post-unsampling of spans in span first

[sentrivana]

Currently, we have SDK as well as integration options that allow to retroactively unsample a trace based on information not available at span start (e.g. status code).

In span first, there is currently no way to sample a span during its lifetime. All existing mechanisms (ignore_spans, sampling) sample at span start, and before_send_span doesn't allow to drop a span.

Clarify whether that's the final design. Once we're clear on the way forward:

• if no form of tail-based sampling is to be supported, add warnings to all options that will be no-op in span first
• if we allow integrations to do some post-sampling with tailored options (like not tracing traces with specific status codes), port the options to work in span first

[linear-code]

PY-2489