Preserve structured properties from existing logging arguments

[Radek Maziarka (rmaziarka)]

Project

• Main issue: Structured properties in Azure Functions Node.js logging.
• Need: Preserve structured properties from existing Azure Functions Node.js context.log/error arguments as separately queryable Application Insights fields.
• Scope:
  • PR 1 in azure-functions-nodejs-worker - carry properties to log hooks.
  • PR 2 in azure-functions-nodejs-library - extract properties from existing arguments with explicit opt-in.
  • PR 3 in azure-functions-nodejs-opentelemetry - export properties and fix the ESM log category.

Problem

• context.error('External service request failed', { service, operation, statusCode }) accepts the object but loses its structure.
• InvocationModel formats all arguments into a string before forwarding the log.
• Existing report: custom properties in context.log, issue #227.

Proposed solution

• Keep context.log/info/warn/error(...args: unknown[]); no new logging method.
• Explicit startup opt-in; proposed option: structuredLogProperties: 'lastPlainObject'.
• Extract properties from the final argument when it is a plain object, including a null-prototype object.
• Preserve format(...args) exactly; the object remains in the message and additionally supplies structured fields.
• Flat strings, booleans and finite numbers; skip unsupported values and reserved keys with bounded diagnostics.
• Forward properties through PR 1's metadata contract; expose them to log hooks.
• Default disabled; unsupported worker retains the existing message and produces a warning.
• Tests: opt-in, unchanged formatting, scalar validation, JavaScript edge cases and old-worker behavior.
• Dependency: released worker capability and core types from PR 1.

Validation

• Core type declarations and the registration adapter restrict structured logging options to log hooks.

• 471 tests passed, including public type checks. Build and lint passed.

• Combined integration test: local Functions host, modified packages and live Application Insights; 30 records verified across CommonJS, ESM, disabled opt-in, missing consumer and old-worker scenarios.

• Verified message preservation, custom dimensions, severity and invocation correlation. Not tested in managed Azure Functions; rollout requires coordinated releases.

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