Such usage reflects the earlier use of the term "canon" in reference to classical Greek authors: the canon of Greek poets, or the canon of Greek orators. How early is this use of the word "canon"?
In his book on the New Testament canon, Bruce Metzger has a very helpful appendix on the development of the word canon (and you can read the whole thing here). At the bottom of p. 289, Metzger mentions the use of the Greek word κανών by Aristotle and others with the significance of "criterion" or some such, not in reference to literary works. Then Metzger says:
With reference to literature and style, the grammarians of Alexandria gave the name κανών to the collection of classical works deemed worthy of being followed as models because of the purity of their language. (pp. 289–90)Metzger goes on to give further examples of "canon" meaning "standard," but not in reference to literature, but in reference to spears or music or epochs or whatever.
But Metzger was wrong about the earliest use of the word "canon" in reference to literature. The grammarians of Alexandria did not, in fact, use the word κανών for any collection of classical works. Such usage of "canon" came only very much later, in 1768, in a work of David Ruhnken. Here's the citation: David Ruhnken, “Historia critica oratorum Graecorum,” in P. Rutilii Lupi: De Figuris Sententiarum et Elocutionis Duo Libri (Lyons: Samuel and Joannes Luchtmans, 1768), xxxiii–c, at xcv. And this one you can also see online.
Here's the description of this development by Rudolph Pfeiffer in the first volume of his History of Classical Scholarship (Oxford, 1968). He begins talking about the way classical Greek works were labeled by the Alexandrian grammarians.
The complete repertories were called πίνακες (indexes); but there was no corresponding Greek or Latin word for the selective lists. In the year A.D. 1768 the term 'canon' was coined for them by David Ruhnken, when he wrote: 'Ex magna oratorum copia tamquam in canonem decem dumtaxat rettulerunt' (sc. Aristarchus et Aristophanes Byzantius). Then Ruhnken dropped the cautious 'tamquam' and went on calling all the selective lists 'canones'. His coinage met with worldwide and lasting success, as the term was found to be so convenient; one has the impression that most people who use it believe that this usage is of Greek origin. But κανών was never used in this sense, nor would this have been possible. From its frequent use in ethics κανών always retained the meaning of rule or model. Aristophanes' grammatical observations about analogy in declension could be called κανόνες, rules, or a certain author and his style could be described as κανών, a model or exemplar. So it was not by the ancient, but it could have been by the Biblical, tradition that the catachrestic use of canon was suggested to Ruhnken. Though the Biblical canon does not mean a list of writers, it does mean a list of books of the Bible accepted by the Christian church as genuine and inspired; and this usage was and is current in all the modern languages. (p. 207)Pfeiffer correctly refers to the usage of "canon" in reference to the Bible. This usage does go back to antiquity, to the work by Athanasius called De Decretis 18.3, written about the year 350, in which Athanasius says that the Shepherd of Hermas is not in the canon. More famously, in his 39th Festal Letter from about a decade and a half later (367), Athanasius uses the participle κανονιζόμενος ("canonized") in reference to the list of authoritative books. Since then, the word "canon" has been commonly used in reference to the Bible, but apparently not in reference to other literary works until 1768.
