Implement %ret special parameter for using in #[instrument(fields())]

[codingskynet]

Feature Request

Crates

tracing

Motivation

tracing supports recording returned value by #[instrument(ret)]. But, I think that it is useful that returned value is used in fields arguments. For example, #[instrument(fields(processed_returned = func(%ret))] like adding field based on the function argument.

Proposal

I proposed adding new reserved keyword for returned value like %ret for fields. It can be used for calculating new field from the returned value.

Alternatives

Now, it can be done by manual recoding like Span::current().record("new_field", func(&returned_value)); before returning value.