feat: add valuable serialization support to appender-tracing

[steved]

Fixes #2819
Design discussion issue (if applicable) N/A

Changes

Supports the valuable extension to tracing for appender-tracing. I moved the serde::Serialize extension out of appender-log into the main package in order to share it with appender-tracing, but if that's not ideal, happy to do something else.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 91.72414% with 36 lines in your changes missing coverage. Please review.

Project coverage is 80.4%. Comparing base (0e3511e) to head (e4ff5a4).

Files with missing lines	Patch %	Lines
opentelemetry/src/logs/serde.rs	91.9%	32 Missing ⚠️
opentelemetry-appender-tracing/src/layer.rs	88.5%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3033     +/-   ##
=======================================
+ Coverage   80.1%   80.4%   +0.2%     
=======================================
  Files        126     127      +1     
  Lines      21949   22110    +161     
=======================================
+ Hits       17603   17782    +179     
+ Misses      4346    4328     -18     

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[Copilot]

Pull Request Overview

This PR adds experimental support for valuable serialization to appender-tracing by extracting the serde::Serialize extension into the main package and updating dependencies accordingly.

• Introduces the valuable feature support in the tracing layer.
• Updates Cargo.toml and allowed-external-types.toml to include serde and valuable dependencies.
• Adds a new test to verify the valuable serialization behavior.

Reviewed Changes

Copilot reviewed 9 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
opentelemetry/src/logs/mod.rs	Adds serde serialization support for logs using conditional compilation.
opentelemetry/allowed-external-types.toml	Includes serde::ser::Serialize among allowed external types.
opentelemetry/Cargo.toml	Adds serde dependency and a new "with-serde" feature.
opentelemetry-appender-tracing/src/layer.rs	Implements serialization for valuable support and adds a corresponding test.
opentelemetry-appender-tracing/Cargo.toml	Introduces valuable and valuable-serde dependencies and updates feature mapping.
opentelemetry-appender-tracing/CHANGELOG.md	Updates changelog to document the new valuable support feature.
opentelemetry-appender-log/Cargo.toml	Adjusts the with-serde feature dependencies to reference the updated serde support.
Cargo.toml	Adds an unexpected_cfgs lint for tracing_unstable builds to the workspace configuration.
.cargo/config.toml	Sets the rustflags to enable tracing_unstable configuration.

Comments suppressed due to low confidence (1)

opentelemetry-appender-tracing/src/layer.rs:187

• Consider adding an inline comment here clarifying the rationale behind using the debug formatted value as a fallback when serialization fails.

            }

[TommyCpp]

Wrong PR number? maybe forget to update after open the PR?

[TommyCpp]

can we prefix it with experimental_ like other experimential features?

[TommyCpp]

Why do we want debug printout here?

[TommyCpp]

Can we also include information of which types are we trying to serialize

[cijothomas]

could you add tests with deeper nested structures? is there any limit to the depth? (Spec is still unclear on how to handle this, but want to make sure we have some limits.)