docs: reconcile OTLP fan-out env var and sanitize package visibility

[Copilot]

Nightly docs reconciliation found two user-facing gaps: GH_AW_OTLP_ENDPOINTS was implemented but undocumented in public docs, and internal/sanitize/ was missing from contributor-facing project structure docs. This PR aligns documentation with current behavior and package layout.

• Tracing env var docs (user-facing)

  • Added GH_AW_OTLP_ENDPOINTS to:
    • README.md Tracing table
    • docs/ENVIRONMENT_VARIABLES.md OpenTelemetry/Tracing variables table
  • Documented:
    • supported formats (comma-separated URLs, JSON array with per-endpoint headers)
    • fan-out semantics (all endpoints receive spans)
    • precedence over OTEL_EXPORTER_OTLP_ENDPOINT
    • shared-header behavior for comma-separated entries

• Contributor docs: project structure completeness

  • Updated CONTRIBUTING.md to include internal/sanitize/ in:
    • project tree
    • Key Directories section
  • Added concise package purpose: secret redaction helpers for safe logging.

• Documentation snippet added

  | `GH_AW_OTLP_ENDPOINTS` | Comma-separated OTLP endpoint URLs, or a JSON array of `{ "url", "headers?" }` objects, for multi-backend fan-out tracing. When set, spans are exported to all listed endpoints (partial-failure tolerant). Takes precedence over `OTEL_EXPORTER_OTLP_ENDPOINT`. Shared headers from `OTEL_EXPORTER_OTLP_HEADERS` / config apply to comma-separated entries. | (none) |

[Copilot]

Pull request overview

This PR reconciles public and contributor documentation with existing gateway behavior by documenting the GH_AW_OTLP_ENDPOINTS tracing fan-out environment variable and by adding the missing internal/sanitize/ package to the documented project structure.

Changes:

• Documented GH_AW_OTLP_ENDPOINTS in the README tracing section.
• Added GH_AW_OTLP_ENDPOINTS to docs/ENVIRONMENT_VARIABLES.md with supported formats and precedence details.
• Updated CONTRIBUTING.md project structure docs to include internal/sanitize/ and its purpose.

Show a summary per file

File	Description
README.md	Adds GH_AW_OTLP_ENDPOINTS to the tracing environment variable table.
docs/ENVIRONMENT_VARIABLES.md	Adds detailed GH_AW_OTLP_ENDPOINTS documentation to the tracing variables table.
CONTRIBUTING.md	Updates the documented internal/ directory tree and key directories list to include internal/sanitize/.

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• Files reviewed: 3/3 changed files
• Comments generated: 1

[lpcox]

@copilot address review feedback

[Copilot]

@copilot address review feedback

Addressed in 650176e. I updated docs/ENVIRONMENT_VARIABLES.md to document that shared OTLP headers apply to all GH_AW_OTLP_ENDPOINTS entries, with JSON per-endpoint headers overriding on key conflicts, and clarified the section as OpenTelemetry plus gateway-specific tracing variables.