(Openinference Migration: Langchain) Add support for legacy OpenAI function call (PR #2)

[rads-1996]

Description

Part of #272

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How has this been tested?

Please describe the tests that you ran to verify your changes. Provide
instructions so we can reproduce. List any relevant details for your test
configuration.

• uv run tox -e py312-test-instrumentation-genai-langchain -- -q
• uv run tox -e py312-test-instrumentation-genai-langchain-conformance -- -q
• uv run --python 3.12 tox -e lint-instrumentation-genai-langchain

Checklist

See CONTRIBUTING.md
for the style guide, changelog guidance, and more.

• Followed the style guidelines of this project
• Changelog updated if the change requires an entry
• Unit tests added
• Documentation updated

[opentelemetry-pr-dashboard]

Pull request dashboard status

Status last refreshed: 2026-07-25 18:45:09 UTC.

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[Copilot]

Pull request overview

Adds support in the LangChain GenAI instrumentation for legacy (pre-tool_calls) OpenAI additional_kwargs["function_call"] so it surfaces as a ToolCallRequest in both input and output message models, aligning behavior with the modern tool_calls path.

Changes:

• Added _legacy_function_call_request() helper to parse additional_kwargs["function_call"] (including JSON-string arguments) into a ToolCallRequest.
• Extended both _ai_message_parts() (input path) and on_llm_end() (output path) to emit the legacy function call as a tool-call request part.
• Added unit tests covering dict/string arguments, absence behavior, and end-to-end callback handler output message behavior; added a towncrier changelog fragment.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
instrumentation/opentelemetry-instrumentation-genai-langchain/tests/test_callback_handler.py	Adds regression/unit tests validating legacy function_call is converted into ToolCallRequest in input/output message modeling.
instrumentation/opentelemetry-instrumentation-genai-langchain/src/opentelemetry/instrumentation/genai/langchain/utils.py	Implements legacy function_call extraction and wires it into AI message part construction.
instrumentation/opentelemetry-instrumentation-genai-langchain/src/opentelemetry/instrumentation/genai/langchain/callback_handler.py	Emits legacy function_call as an output tool-call request part during on_llm_end.
instrumentation/opentelemetry-instrumentation-genai-langchain/.changelog/281.added	Documents the added legacy function_call capture support.