Support constant expressions as instrument field names

[nico-incubiq]

Motivation

The #[instrument] proc macro only allows setting field names as identifiers, which is not on par with other macros such as span! or event! that allow constant expressions. Using constants for example can ensure consistency across a project for the field names used.

This addresses #2969, which was already mentioned in this comment here on #2617 (note that "classic macros" mentioned there already work, there are tests attesting of it).
I also updated the doc block to mention field values can be any arbitrary expressions, which was implemented by #672 but never updated.

Solution

I updated the instrument proc macro to allow constant expressions (const, const fn, literal str, ...) as field names, using the same syntax as #2617, enclosed in curly braces.

const CONST_FIELD_NAME: &str = "foo.bar";

#[instrument(fields({CONST_FIELD_NAME} = "quux"))]
fn fn_const_field_name() {}

Notes

• The identifier (Punctuated) field names have a convenient feature allowing to supersede non-skipped function arguments (a field named s would be used instead of a function argument s in the span). As far as I'm aware this is not possible to implement for constant expressions, as the compiler doesn't allow to evaluate constants at build time (MIRI might be a solution but that seems complicated, and https://rustc-dev-guide.rust-lang.org/const-eval seems experimental). It's a minor inconsistency; in this scenario, the code would panic immediately at startup, and the workaround is as simple as skipping the function argument.

[nico-incubiq]

all of this is because the code is executed from a const fn context. i couldn't find off-the-shelf functions that do the same, and I also couldn't find a better place to add this check than the constructor of the FieldSet.