tracer.go
func (r SpanReference) Apply(o *StartSpanOptions) {
	if r.ReferencedContext != nil {
		o.References = append(o.References, r)
	}
}
func (r SpanReference) Apply(o *StartSpanOptions) {
	if r.ReferencedContext != nil {
		o.References = append(o.References, r)
	}
}
// Apply satisfies the StartSpanOption interface.