ext/tags.go
func (tag ipv4Tag) SetString(span opentracing.Span, value string) {
	span.SetTag(string(tag), value)
}
func (tag ipv4Tag) SetString(span opentracing.Span, value string) {
	span.SetTag(string(tag), value)
}
// SetString records IP v4 host address of the peer as a .-separated tuple to the `span`. E.g., "127.0.0.1"