Here getting people ready to welcome Yahweh is spoken of as if a road were being cleared for Yahweh to travel on. (See: [[rc://en/ta/man/translate/figs-metaphor]])
Some modern versions translate this in a way that implies that these two expressions refer to the same person. Other modern versions leave this matter ambiguous. We recommend that translations leave this matter ambiguous, as the ULB and UDB do.
Almost all versions leave ambiguous the sense of this expression. But translators may need to make explicit the relationship between "messenger" and "the covenant." The UDB presents "the messenger" as one promised by the covenant that Yahweh had with Israel. Another choice is to present the messenger as a person who will either confirm that covenant or announce a new covenant.