Prepare for 0.4.21 release

[KodrAus]

cc @Thomasdezeeuw @JohnTitor This release stabilizes our structured logging API.

Closes #616

What's Changed

• Minor clippy nits by @nyurik in Minor clippy nits #578
• Simplify Display impl by @nyurik in Simplify Display impl #579
• Set all crates to 2021 edition by @nyurik in Set all crates to 2021 edition #580
• Various changes based on review by @Thomasdezeeuw in Various changes based on review #583
• Fix typo in file_static() method doc by @dimo414 in Fix typo in file_static() method doc #590
• Specialize empty key value pairs by @EFanZh in Specialize empty key value pairs #576
• Fix incorrect lifetime in Value::to_str() by @peterjoel in Fix incorrect lifetime in Value::to_str() #587
• Remove some API of the key-value feature by @Thomasdezeeuw in Remove some API of the key-value feature #585
• Add logcontrol-log and log-reload by @swsnr in Add logcontrol-log and log-reload #595
• Add Serialization section to kv::Value docs by @Thomasdezeeuw in Add Serialization section to kv::Value docs #593
• Rename Value::to_str to to_cow_str by @Thomasdezeeuw in Rename Value::to_str to to_cow_str #592
• Clarify documentation and simplify initialization of STATIC_MAX_LEVEL by @ptosi in Clarify documentation and simplify initialization of STATIC_MAX_LEVEL #594
• Update docs to 2021 edition, test by @nyurik in Update docs to 2021 edition, test #577
• Add "alterable_logger" link to README.md by @brummer-simon in Add "alterable_logger" link to README.md #589
• Normalize line ending by @EFanZh in Normalize line ending #602
• Remove ok_or in favor of Option::ok_or by @AngelicosPhosphoros in Remove ok_or in favor of Option::ok_or #607
• Use Acquire ordering for initialization check by @AngelicosPhosphoros in Use Acquire ordering for initialization check #610
• Get structured logging API ready for stabilization by @KodrAus in Get structured logging API ready for stabilization #613

[nyurik]

Should this be a minor change? I think we need to add two lines to the CI workflow as described here:

- name: Check semver
  uses: obi1kenobi/cargo-semver-checks-action@v2

[nyurik]

I just added #618 - it might need to go in first to make sure we are not accidentally breaking semver contract

[nyurik]

P.S. yes, it is clearly failing, possibly due to the current Cargo.toml having the same version as the published one. It seems patch version bump is not enough though.

My practice with that checker is that the version in the github main branch should be changed as soon as the breaking change is introduced. So either keep it as 0.5.0-pre.1 or as 0.5.0, but in either case it must not be 0.4.20 after the release and after the breaking changes are merged.

[Thomasdezeeuw]

Should this be a minor change?

What breaking changes do we have?

I think we need to add two lines to the CI workflow as described here:

Let's discuss this in the other pr.

[nyurik]

@Thomasdezeeuw I see a bunch of the breaking changes in https://github.com/rust-lang/log/actions/runs/8072374937/job/22053982534?pr=618 -- do you think they are not correct?

[Thomasdezeeuw]

@Thomasdezeeuw I see a bunch of the breaking changes in https://github.com/rust-lang/log/actions/runs/8072374937/job/22053982534?pr=618 -- do you think they are not correct?

Looking at that output. The as_map and as_list related stuff was removed on purpose.
Two things renamed: Key::to_borrowed_str -> Key::as_str and Value::to_str -> Value::to_cow_str.

So it LGTM, all the changes were part of the unstable API.

[KodrAus]

Shipping this now 👍 I'll keep an eye on it over the next little while because releases with macro changes always come with risk.

[KodrAus]

@Thomasdezeeuw I've invited you to the organization I created where value-bag lives and will add you to the crate publishers too. Don't feel compelled to do anything if you're not interested (although you're very welcome to if you'd like!), but having permission to get builds and patches through there could be really useful someday.