Enable suppport for Valuable in instrument macro

[software-artificer]

Extend the instrument macro to support recording the return and the error values using the valuable::Valuable implementation by writing ret(Valuable) and err(Valuable) respectively.

Motivation

Tracing already comes with valuable support today for various fields. This can be either done through value.as_value() directy or by means of calling tracing::field::valuable(value). However, it does not currently support recording return or error fields when using the #[instrument(err)] or #[instrument(ret)] values. Proposed change introduces this support.

This will further be used by the tracing-opentelemetry to record the exception.type field when the error value implements the Valuable trait and will allow serialization with Valuable for return types.

Solution

The proposal here is pretty simple: add support for err(Valuable) and ret(Valuable), guarded behind the valuable feature flag and also hidden behind the unstable cfg as per current tracing implementation.

Additional context in Tokio Discord.