Function set_remote_parent 

pub fn set_remote_parent<'a, I, V>(span: &Span, headers: I)
where
    I: IntoIterator<Item = (&'a str, V)>,
    V: AsRef<[u8]>,

Make span a child of the trace that headers carries.

The headers hold the producer’s context, so consumer-side work then appears in the same distributed trace. This function does nothing when the headers carry no valid trace context.