This imaginary concept derives from this text in T'hillim:
תְּחַסְּרֵהוּ מְּעַט, מֵאֱלֹהִים
So the article 'METATRON' in Jewish Encyclopedia:
'The Zohar defines his nature exactly by declaring that he is little lower than God (after Ps. viii. 6; Yalḳ. Ḥadash, 7, No. 51).'
Though the Targum translates מֵאֱלֹהִים in T'hillim 8:6 as 'angels,' Aquila translates this as theos ('God').
The title you mention comes from 3 Enoch 12.5. Enoch says in 4.3 that he had been taken from earth to heaven, and in 12.5 he narrates the giving of the title to him.
God clothes Metatron in a garment of glory, puts a royal crown on his head and calls him "the Lesser YHWH."
R. Ishmael said: Metatron, the Prince of the Presence, said to me:
(1) By reason of the love with which the Holy One, blessed be He, loved me more than all the
children of heaven. He made me a garment of glory on which were fixed all kinds of lights, and He
clad me in it.
(2)And He made me a robe of honour on which were fixed all kinds of beauty, splendour, brilliance
(3) And he made me a royal crown in which were fixed forty-nine costly stones like unto the light of the globe of the sun.
(4) For its splendour went forth in the four quarters of the Araboth Raqia', and in (through) the seven heavens, and in the four quarters of the world. And he put it on my head.
(5) And He called me THE LESSER YHWH in the presence of all His heavenly household; as it is written (Ex. xxiii. 21): "For my name is in him".