But without the actual sign by God, the rest is useless.
Without that sign Ahaz would not have been given confirmation of Gods promise, not only to his house, but to his royal line and by extension to the whole of Israel. Think about it, here we have a king deeply worried, he has no successor (no children), he is dying, Israel is being threatened with invasion by the king of Assyria, if this sign is a metaphor, but the actual dialogue is real, what kind of confirmation would have been enough for Ahaz?
Josephus in Antiquities 10, 2 gives an an account of this passage, which interestingly enough is not considered to be metaphorical by him.
1. NOW king Hezekiah being thus delivered, after a surprising manner, from the dread he was in, offered thank-offerings to God, with all his people, because nothing else had destroyed some of their enemies, and made the rest so fearful of undergoing the same fate that they departed from Jerusalem, but that Divine assistance. Yet, while he was very zealous and diligent about the worship of God, did he soon afterwards fall into a severe distemper, insomuch that the physicians despaired of him, and expected no good issue of his sickness, as neither did his friends: and besides the distemper itself, there was a very melancholy circumstance that disordered the king, which was the consideration that he was childless, and was going to die, and leave his house and his government without a successor of his own body; so he was troubled at the thoughts of this his condition, and lamented himself, and entreated of God that he would prolong his life for a little while till he had some children, and not suffer him to depart this life before he was become a father. Hereupon God had mercy upon him, and accepted of his supplication, because the trouble he was under at his supposed death was not because he was soon to leave the advantages he enjoyed in the kingdom, nor did he on that account pray that he might have a longer life afforded him, but in order to have sons, that might receive the government after him. And God sent Isaiah the prophet, and commanded him to inform Hezekiah, that within three days' time he should get clear of his distemper, and should survive it fifteen years, and that he should have children also. Now, upon the prophet's saying this, as God had commanded him, he could hardly believe it, both on account of the distemper he was under, which was very sore, and by reason of the surprising nature of what was told him; so he desired that Isaiah would give him some sign or wonder, that he might believe him in what he had said, and be sensible that he came from God; for things that are beyond expectation, and greater than our hopes, are made credible by actions of the like nature. And when Isaiah had asked him what sign he desired to be exhibited, he desired that he would make the shadow of the sun, which he had already made to go down ten steps [or degrees] in his house, to return again to the same place, and to make it as it was before. And when the prophet prayed to God to exhibit this sign to the king, he saw what he desired to see, and was freed from his distemper, and went up to the temple, where he worshipped God, and made vows to him.
What is also interesting is that this event was also mentioned elswhere in the bible, specifcally 2 Chronicles 32:24, 31.
24 In those days Hezekiah became ill and was at the point of death. He prayed to the Lord, who answered him and gave him a miraculous sign.
31 But when envoys were sent by the rulers of Babylon to ask him about the miraculous sign that had occurred in the land, God left him to test him and to know everything that was in his heart.
Now, if this a metaphor as you claim, could you tell me what other miraculous signs these envoys would be asking Ahaz about?
Tell me something, have you ever heard of the term "the shekinah of the Lord"?
I propose to you, that what caused the shadow to recede, was nothing more than the Shekinah Glory of the Lord, brighter than any sun...