Function extract_parent_context 

pub fn extract_parent_context(headers: &HeaderMap) -> Context

Extract a parent opentelemetry::Context from the supplied headers via whichever global propagator has been installed (typically TraceContextPropagator after install_w3c_propagator has run).

Returns an empty context if no propagator is installed or if the headers do not carry a recognised trace-context entry; the caller can pass that empty context to set_parent safely — the resulting span becomes a new local root, which is the documented degradation behaviour.