Ways in which Cervantes is metafiction

 1. The work is a parody, so it is automatically metafiction, because of intertextuality.  

2. Breaking the theatrical fourth wall, in DQ's reaction to Maese Pedro's puppet show. 

3. In the second part, commentaries on the 1st part, and critique of the apocryphal Quijote. The printing press in Barcelona. 

4. Discussions of the merits of various literary works. The burning of books at the end of the work. 

5. Interpolated novels. 

6. The found manuscript and translation tropes.  

There are probably other things too, depending on how one wants to slice the pie and define these terms.