Allow value_sets of any length

[eopb]

Motivation

fixes: #1566

Solution

This change removes the maximum of 32 fields limitation using const generics.

Having this arbitrary restriction in place to prevent stack overflows feels a little misplaced to me since stack size varies between environments.

[hawkw]

I'm fine with making this change. If memory serves, I believe the motivation for the array length limit was due to Rust compiler limitations that no longer exist.

[hawkw]

oh, before we merge this: there's a docs example stating that a span may only have 32 fields or fewer, which shows creating a span with > 32 fields and asserts that it doesn't compile:

tracing/tracing/src/lib.rs

Lines 389 to 403 in bf05c61

	//! Note that a span may have up to 32 fields. The following will not compile:
	//!
	//! ```rust,compile_fail
	//! # use tracing::Level;
	//! # fn main() {
	//! let bad_span = span!(
	//! Level::TRACE,
	//! "too many fields!",
	//! a = 1, b = 2, c = 3, d = 4, e = 5, f = 6, g = 7, h = 8, i = 9,
	//! j = 10, k = 11, l = 12, m = 13, n = 14, o = 15, p = 16, q = 17,
	//! r = 18, s = 19, t = 20, u = 21, v = 22, w = 23, x = 24, y = 25,
	//! z = 26, aa = 27, bb = 28, cc = 29, dd = 30, ee = 31, ff = 32, gg = 33
	//! );
	//! # }
	//! ```

Let's make sure to remove that example before merging this PR?

---

As a side note, I would expect the inclusion of that example with compile_fail to be breaking the CI build now that it compiles...which makes me concerned that we're not actually running doctests on CI, or something...

[hawkw]

Hmm, actually, it looks like that test did run on this PR, and it did fail to compile: https://github.com/tokio-rs/tracing/actions/runs/6087610980/job/16516577365?pr=2508#step:6:186

This makes me concerned that there's some other reason that test doesn't compile, in addition to the field length limit. Or, we haven't fully removed field length validation. Could we maybe add a new normal test to the macro tests that tries to create a span with > 32 fields, just to make sure that it works?

[eopb]

Hmm, actually, it looks like that test did run on this PR, and it did fail to compile: https://github.com/tokio-rs/tracing/actions/runs/6087610980/job/16516577365?pr=2508#step:6:186

This makes me concerned that there's some other reason that test doesn't compile, in addition to the field length limit. Or, we haven't fully removed field length validation. Could we maybe add a new normal test to the macro tests that tries to create a span with > 32 fields, just to make sure that it works?

error: cannot find macro `span` in this scope
 --> tracing/src/lib.rs:395:16
  |
6 | let bad_span = span!(
  |                ^^^^
  |
help: consider importing this macro
  |
4 + use tracing::span;
  |

error: aborting due to previous error

The test wasn't failing for the right reasons :)

[eopb]

@hawkw Fixed and I've added a test

[hawkw]

fantastic, thank you! let's merge this as soon as CI passes! :)