Can the following sentence be literal (or is it a metaphor)

"To wax a metaphor" may be poetic, but it is not a metaphor. From the OED:

Of a quality, activity, event, etc.: To come into being, spring up, begin, arise, occur. Also with up. Of the day: To appear, dawn.

and

to wax forth, to be born or created.

So, your example Today I learnt how to wax a metaphor means, literally, that Today I learnt how to (create/bring into being/cause to appear or occur) a metaphor. Archaic for sure, but not metaphoric.

A metaphor is a sentence like all the world is a stage which works by viewing the world as having some of the characteristics of a stage, and has the form of the world being spoken of as a stage when it only resembles a stage, which makes it different to a simile which normally uses "as" or "like". Keep in mind that the conceptual view of metaphor as anything that maps the characteristics of A onto B covers a much wider range of forms.

In your example, nothing is being spoken of as being something else, nor is any characteristic being transferred to something else, so it is not a metaphor by either the traditional or the conceptual model.