Metatron is a real and substantial figure. He appears in the Babylonian Talmud, in the Hekhalot literature, and above all in the text known as 3 Enoch or Sefer Hekhalot, where he is the transformed patriarch Enoch, Prince of the Presence, heavenly scribe, and bearer of the startling title the lesser YHWH. That title is provocative enough that the rabbinic sources preserve a famous story of the sage Elisha ben Abuyah seeing him enthroned and concluding, wrongly and catastrophically, that there were two powers in heaven. Metatron belongs squarely to the world of Merkabah and Hekhalot mysticism in the early centuries of the common era, and he went on mattering in medieval Jewish esoteric writing.
None of that literature contains this diagram. There is no known classical or medieval Kabbalistic text describing a figure of thirteen circles and seventy-eight lines, and none giving any such figure Metatron's name. Popular accounts asserting that the earliest references to Metatron's Cube are found in medieval Kabbalah do not cite a manuscript, and until one is produced the attribution should be treated as unsupported.
What can be said is that the pairing of the name with the figure belongs to modern Western occultism, and that it reached a mass audience through the sacred-geometry teaching of the later twentieth century, particularly the Flower of Life material from the 1980s onward. The choice is intelligible: a scribe-angel associated with the divine throne and the architecture of the heavens makes a natural patron for a diagram claiming to be a blueprint of creation. But it is an association made recently, by people who had read the older sources, rather than one inherited from them.