Mark publish() as must_use

[suzmue]

Ignoring the result of a publish operation is an antipattern in Pub/Sub. Mark the future as "must_use" so that it needs to be explicitly ignored.

[suzmue]

Because the return type implements future, it would trigger this particular clippy error as well: https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#let_underscore_future