Declare Functions From Target

Because we are linking with our target statically, we need to declare the functions we want to call from it as extern. This has the same meaning as in C/C++, and informs the compiler that we will link with these symbols, but we are not defining them.

Add the following block below the allocator definition.

#![allow(unused)]
fn main() {
extern "Rust" {
    fn decode(encoded_input: &[u8]) -> Vec<u8>;
    fn counters_maps_observer(name: &'static str) -> CountersMultiMapObserver<false>;
}
}