Add blanket implementations of Recorder for some pointer-like types

[tobz]

(I messed up the remote branch in #507 and it was just easier to fix it by making a new branch and PR.)

Rc, Arc, and Box.

Closes #506.

[Nekrolm]

Thanks for implementing this!

Can you also add + ?Sized bound for blanket impl to support Box<dyn Recorder> and similar?

Also I can suggest to use macro to not repeat boilerplate
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021

macro_rules! impl_recorder {
    (<$t:tt> for $ptr:ty) => {
        impl <$t> Recorder for $ptr
        where $t : Recorder + ?Sized 
        {
            fn describe_counter(&self, key: &str) {
                $t::describe_counter(self, key)
            }
        }
    }
}

impl_recorder!(<T> for &T);
impl_recorder!(<T> for &mut T);
impl_recorder!(<T> for Box<T>);
impl_recorder!(<T> for std::sync::Arc<T>);
impl_recorder!(<T> for std::rc::Rc<T>);

[tobz]

Yeah, that's a good idea about reducing the boilerplate with a macro, and also adding blanket implementations for borrowed variants.

It also reminded me that we can't have a blanket implementation for Rc<T> now since Recorder has to be Sync.

[tobz]

Released as part of metrics@v0.24.0/metrics-util@v0.18.0.