tracer.go
func (t Tag) Set(s Span) {
	s.SetTag(t.Key, t.Value)
}
func (t Tag) Set(s Span) {
	s.SetTag(t.Key, t.Value)
}
func ( t Tag ) Set ( s Span ) { s . SetTag ( t . Key , t . Value ) }
// Set applies the tag to an existing Span.
Set applies the tag to an existing Span .