lib: metadata: add missing non-const borrowing accessors

lib: metadata: add missing non-const borrowing accessors

Add a few missing non-const borrowing accessors in the metadata part of
trace IR. They are not strictly needed for anything written in C
(because since you own the non-const object, you typically don't need to
read a property that you set yourself), but they will help make some
binding wrapper APIs symmetrical.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>