I've pushed (and backpatched) a fix for this.
I ended up doing the simplest thing -- error out if the number of
columns does not match, suggesting to update to latest extension version.
I considered handling it in a nicer way, but I didn't like the result
very much and I think that's sufficient for superuser-only extension.
And 691e8b2e18 seems like a reasonable precedent (even though the
backbranches did do a different thing).
I also considered introducing pg_stat_statements-style versioning, but
it's too late to do that in backbranches, and I don't think we expect
the function to change very often to justify this.
regards
--
Tomas Vondra