message: attributes: add visibility to async-trait output
  Instrument removes the visibility of function that looks like its
  from async-trait even if its not inside a trait impl block. This
  adds it back for support of proc-macros based off async-trait.
  Fixes:
diff: @@ -285,11 +285,12 @@ pub fn instrument(
  }
  }
  + let vis = &input.vis;
  let sig = &input.sig;
  let attrs = &input.attrs;
  quote!(
  #(#attrs) *
  - #sig {
  + #vis #sig {
  #(#stmts) *
  }
  )
  