enhancement: add environment-backed exporter headers to direct APIs

[ericevans-nv]

Overview

Direct OpenTelemetry subscribers accepted resolved static headers, but did not provide the environment-backed header configuration already available to declarative exporter configuration.

This adds consistent header_env support to direct trace, log, and metric APIs across Rust, Python, Node.js, and Go. Environment values are resolved once when the subscriber is constructed and are used only as outbound exporter headers.

• I confirm this contribution is my own work, or I have the right to submit it under this project's license.
• I searched existing issues and open pull requests, and this does not duplicate existing work.

Details

• Map each outbound header name to the environment variable supplying its value.
• Resolve and snapshot environment-derived values during subscriber construction.
• Preserve existing static headers behavior.
• Reject missing, blank, padded, or invalid environment references and values.
• Reject duplicate header names across headers and header_env, including ASCII case collisions.
• Keep resolved values out of telemetry payloads, Event data, resource attributes, errors, and runtime diagnostics.
• Add equivalent direct configuration surfaces for Rust, Python, Node.js, and Go.
• Add versioned C FFI constructors while preserving existing constructor compatibility.
• Avoid exposing low-level environment read details in declarative plugin errors.
• Document the direct API lifecycle, snapshot behavior, validation rules, and secret-handling boundary.

Validation completed against the committed implementation:

• Focused Rust header_env tests: 11 passed.
• Real localhost OTLP HTTP exporter request test: passed and verified the activation-time header snapshot, nonempty payload, and secret non-disclosure.
• Versioned FFI constructor tests: passed.
• Focused Python tests: 5 passed.
• Focused Node.js constructor, request, and public type tests: passed.
• Focused Go subscriber and localhost request tests: passed.
• File-scoped pre-commit checks: passed, including Rust formatting, Clippy, checks, FFI header synchronization, Python lint/type checks, Go formatting/vet, Node formatting, and documentation links.
• Documentation build: passed with zero errors.

PR-readiness limitation: just test-rust stopped in the unchanged nemo-relay-cli --lib test surface while a Cursor-managed Relay listener was active. The remaining full language matrix and all-file pre-commit were not repeated under the approved validation waiver.

Where should the reviewer start?

Start with crates/core/src/observability/otel_signal.rs, which implements the shared validation and activation-time resolution contract. Then review crates/core/tests/unit/observability/otel_tests.rs for the real localhost request, snapshot, collision, invalid-value, and non-disclosure coverage.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Closes [Enhancement]: Provide consistent environment-backed exporter headers across bindings #799
• Relates to RELAY-746

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Fern docs preview: https://nvidia-preview-pull-request-901.docs.buildwithfern.com/nemo/relay/dev