All scriptural references are in other answers but here is a thought:
Jesus said that no man has at any time seen Yhwh. Unless Jesus was a liar, this means that all references to Elohiym which are a plural reference to both Yhwh and Yeshua simultaneously according to John 1. Hermaneutically sound teaching combining both Old and New testaments to speak with the same breath says that this is a most likely understanding. Where the Son of Man had not been revealed in the old testament, he kown just as God's son (referenced in Psalms and Job if I recall). Jesus himself declared that he pre-existed Abraham and knew him. With these latter predicates, it may well be possible that Jesus was indeed the King of Peace.
When Abram saw three men approach him from a distance, he ran out to greet them. This indicates that Abram not only saw the men but recognised them. This could have only occured if Abram had previous;y met at least one of the men but the one whom Abram acknowledged was neither servant but their master whom he recognised on sight because he had met him in person previously. This was before he had been blessed by God and before the covenant was made between God and Abram.
It had not been much earlier that Abram had paid a tithe to the King of peace and it would not be unreasonable to consider that the person Abram recognised as the King of Salem with two of his servants were the same person we know as Jesus today. Archaeologists have not found any evidence of this place called Salem and the idea that it grew and became Jeru'Salem is not implausible either since the time covers great swathes of undocumented historical change in reference to the accounts. Abram probably paid a tithe because he had brought spoils of war through the land of peace, under the dominion of Melchisidek, and it would have been dishonourable to not do so. There is no plausible reason in modern understanding for a victorious nation to share the spoils of war with anyone else because it is not in westrn tradition or culture to do so.
In summary, it appears to me that Lord Melchisidek was very possibly Jesus himself. Being sent as the great ambassador of the Most High God, Melchisidek went with two angels to see Abram. Abraham recognised Melchisidek by sight and fell down at his feet to worship him. That is a very significant event because Abraham would not bow down to any man or idol pointing further to the evidence that this was indeed the son of God himself. Melchisidek left Abram as Abraham and sent the two angels ahead to Sodom & Gomorrah in order to destroy them but Lot was found in Sodom to be righteous. The two angels evacuate Lot then destroy the cities. What we do not know is what happened to the third person, tacitly well known by Abraham personally, after meeting with Abraham because the angels went onwards without him with specific instructions from this third person. This is the clue: the angels were the third person's witnesses and junior in rank to him. If it could not possibly have been the Yhwh himself, then it would have been his most trusted amabssador, Jesus, because there are no greater angels than Michael and Gabriel who are the two witnesses constantly before Most High God. This is a case of written evidence that is specifically not specified and answers many hermaneutics of New Testament ideologies and solves some tough issues of prophecy understanding in both testaments that Jesus has always been the king of Jerusalem and that the prophetic life of Hosea represented closely the life of Christ from the beginning of man to the Last Days. With that understanding we are able to better comprehend the meanings of several other prophecies that clarify darker and difficult texts in Revelation and the Prophets.
Was he metaphorical? No, certainly not. He presaged what was to come and even hralded the plan of God from the beginning. Think of the two testaments as being one for Judaeans and the other for the Lost Tribes of Israel. Only those whose names are written in the book of life will be saved and understand these secret things of God. To all others it will likely remain mysterious and conjectural in nature to their thinking.
////an even older testimony is that Adam and Eve knew what God looked like physically and recognised his voice when they heard him in the Garden east of Eden. This too may have been Jesus because no man has ever seen God at any time.////