func (t *Tracer) Tag(key string, value interface{}) {
	t.Last().SetTag(key, value)
}
func (t *Tracer) Tag(key string, value interface{}) {
	t.Last().SetTag(key, value)
}
func ( t * Tracer ) Tag ( key string , value interface { } ) { t . Last ( ) . SetTag ( key , value ) }
// Tag adds a tag to the last pushed span.
Tag adds a tag to the last pushed span .