Function decide 

pub fn decide(
    cfg: &SamplingConfig,
    full_name: &str,
    severity: Severity,
    inbound_sampled: Option<bool>,
) -> SamplingDecision

Run the head sampler. inbound_sampled is the traceparent.sampled bit lifted off the active scope frame; pass None when no scope is active or the caller did not propagate W3C trace context.