feat(api): add experimental env var propagators

[pellared]

Fixes #3285

Related to open-telemetry/opentelemetry-specification#5142

Changes

This adds experimental support for the OpenTelemetry environment-variable carrier specification so Rust applications can propagate context across parent/child process boundaries without relying on network transports.

Follows https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/env-carriers.md

• Add EnvVarExtractor and EnvVarInjector under opentelemetry::propagation, gated behind otel_unstable
• Normalize environment variable names according to the spec for Get, Set, and Keys
• Keep the carrier format-agnostic and value-opaque; propagators remain responsible for parsing and validation
• Add EnvVarExtractor::from_fields as the recommended extraction path, reading only the normalized environment variables advertised by the active propagator
• Keep EnvVarExtractor::from_os_entries as the full-scan fallback for propagators that need keys() over the whole carrier
• Keep EnvVarInjector focused on injected propagation variables and make it consumable by std::process::Command::envs
• Document the API in crate docs and README, and add examples/env-var-propagation showing parent/child process propagation with TraceContextPropagator

Design notes

EnvVarExtractor stores environment values internally because Extractor::get returns borrowed &str values, while std::env::var_os returns owned values. Storing the selected values gives the carrier stable storage to borrow from and gives keys() a consistent view.

Most propagators only need direct get() calls for known fields, so from_fields(propagator.fields()) avoids enumerating the entire process environment in the common case. from_os_entries(std::env::vars_os()) remains available for propagators that scan carrier keys, such as legacy prefix-based formats.

EnvVarInjector does not mutate the parent process environment. Callers inject into a fresh carrier and pass it to Command::envs; Command continues to inherit the rest of the parent environment by default.

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

❌ Patch coverage is 98.67257% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.3%. Comparing base (54c3bb2) to head (8abb673).

Files with missing lines	Patch %	Lines
opentelemetry/src/propagation/env.rs	98.6%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3574     +/-   ##
=======================================
+ Coverage   83.2%   83.3%   +0.1%     
=======================================
  Files        130     131      +1     
  Lines      28231   28457    +226     
=======================================
+ Hits       23491   23713    +222     
- Misses      4740    4744      +4     

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[Copilot]

Pull request overview

Adds experimental support in the opentelemetry API crate for using environment variables as a TextMapPropagator carrier, enabling parent/child-process context propagation per the OpenTelemetry env-carriers specification (behind the otel_unstable feature).

Changes:

• Introduces EnvVarExtractor and EnvVarInjector carriers under opentelemetry::propagation gated by otel_unstable, including key normalization per spec.
• Updates crate docs/README and changelog to document the new experimental API and feature flag.
• Adds a new end-to-end example (examples/env-var-propagation) demonstrating propagation across a real process boundary.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
opentelemetry/src/propagation/mod.rs	Exposes the new env-var carrier module and re-exports the new types under otel_unstable.
opentelemetry/src/propagation/env.rs	Implements the env-var extractor/injector carriers and normalization logic, plus unit tests.
opentelemetry/src/lib.rs	Documents the new example link and clarifies what otel_unstable enables.
opentelemetry/README.md	Adds user-facing documentation pointing to the env-var propagation helpers and example.
opentelemetry/CHANGELOG.md	Records the new experimental API under vNext.
opentelemetry/Cargo.toml	Adds the otel_unstable feature and temp-env dev-dependency for tests.
examples/env-var-propagation/src/main.rs	New runnable example that injects context into child env vars and extracts in the child process.
examples/env-var-propagation/README.md	Documents the example behavior and how to run it.
examples/env-var-propagation/Cargo.toml	New example crate manifest enabling otel_unstable on opentelemetry.

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

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

[pellared]

https://github.com/open-telemetry/opentelemetry-rust/actions/runs/28400305177/job/84149540890?pr=3574 looks like a transient error

[github-actions]

Thank you for your contribution! This PR has been automatically marked as stale because it has not had activity in the last 14 days. This may be due to a delay in review on our side or awaiting a response from you; either is fine, and we appreciate your patience.

It will be closed in 14 days if no further activity occurs. Pushing a new commit or leaving a comment will remove the stale label and keep the PR open.

[kamphaus]

Env var carriers are now RC:

• Mark env var carrier as RC opentelemetry-specification#5142