pub fn message_attribute<P: AsRef<str>, A: AsRef<str>>(
        mut self,
        path: P,
        attribute: A,
    ) -> Self {
        self.message_attributes
            .push((path.as_ref().to_string(), attribute.as_ref().to_string()));
        self
    }
Add additional attribute to matched messages.

Passed directly to `prost_build::Config.message_attribute`.
tonic-build/src/prost.rs