feat(lambda): read cloud.account.id from symlink in Lambda detector

[RaphaelManke]

Summary

• Reads cloud.account.id from the symlink at /tmp/.otel-aws-account-id created by the OTel Lambda Extension
• Uses Files.readSymbolicLink() to read the symlink target
• Logs at FINE level if the symlink doesn't exist (catch IOException | UnsupportedOperationException)

Depends on

• feat(lambda): write cloud.account.id symlink from extension opentelemetry-lambda#2127 (extension creates the symlink)

Changes

• LambdaResource.java — Added readlink logic with ACCOUNT_ID_SYMLINK_PATH constant, injectable Path for testing
• LambdaResourceTest.java — Happy path, missing symlink, leading zeros tests

Test plan

• Unit test: symlink exists → cloud.account.id set correctly
• Unit test: leading zeros preserved as string
• Unit test: missing symlink → attribute absent, no error

[Copilot]

Pull request overview

This PR adds support for reading the AWS account ID from a symlink created by the OpenTelemetry Lambda Extension. The Lambda resource detector now reads cloud.account.id from /tmp/.otel-aws-account-id, enabling automatic population of this attribute without requiring environment variables.

Changes:

• Added symlink reading logic to LambdaResource with graceful error handling
• Introduced injectable Path parameter for testability while maintaining backward compatibility
• Added comprehensive unit tests covering happy path, missing symlink, and leading zero preservation

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
aws-resources/src/main/java/io/opentelemetry/contrib/aws/resource/LambdaResource.java	Added symlink reading logic with proper error handling, maintained backward compatibility with existing buildResource() method signature
aws-resources/src/test/java/io/opentelemetry/contrib/aws/resource/LambdaResourceTest.java	Added three test cases covering symlink reading success, missing symlink graceful handling, and leading zero preservation
CHANGELOG.md	Documented the new feature under the Unreleased section