B3 single-header debug extraction should imply sampled #3810

Description

opentelemetry-propagator-b3 mishandles a valid single-header B3 debug context such as:

b3: 4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-d

With Propagator::new(), extraction returns an invalid context because handling of d incorrectly depends on the configured injection encoding. With single-header injection enabled, the context is extracted as debug but not sampled, so a parent-based sampler can drop child spans.

The B3 specification defines debug as an emphasised accept decision, so d must imply sampled.

Expected behaviour

Single-header d is extracted for every injection configuration as debug plus sampled. X-B3-Sampled: d remains invalid because multiple-header debug uses X-B3-Flags: 1.

Related

A fix and regression test were proposed in #3752.