Ensure trace noop structs use new

[jtescher]

Add private fields to trace module's noop structs to be consistent with this crate's conventions around backwards compatibility.

[frigus02]

👍

[TommyCpp]

Looks good 👍. This should work, but is there a reason why we don't use #[non_exhaustive] in this case?

[jtescher]

@TommyCpp we don't want to allow matching at all, rather than merely non exhaustive matching, but this is more about the constructor specifically. By default constructors via NoopSpan {} are public, this change makes users go through NoopSpan::new() instead so if we want to add some state or otherwise change the internals this makes it not a breaking change.