[opentelemetry-instrumentation-genai-langchain] Capture top_k and choice count on chat

[rads-1996]

Description

Fixes #581

Type of change

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• 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?

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instructions so we can reproduce. List any relevant details for your test
configuration.

• Test A

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

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Pull request overview

Adds LangChain chat extraction for top_k and requested choice count.

Changes:

• Captures top_k and n from invocation parameters.
• Adds unit tests for populated and missing values.
• Adds a changelog fragment.

File summaries

File	Description
instrumentation/opentelemetry-instrumentation-genai-langchain/tests/test_callback_handler.py	Updated as part of this pull request.
instrumentation/opentelemetry-instrumentation-genai-langchain/src/opentelemetry/instrumentation/genai/langchain/callback_handler.py	Updated as part of this pull request.
instrumentation/opentelemetry-instrumentation-genai-langchain/.changelog/684.added	Updated as part of this pull request.

Review details

Suppressed comments (1)

instrumentation/opentelemetry-instrumentation-genai-langchain/src/opentelemetry/instrumentation/genai/langchain/callback_handler.py:305

• Please keep this consistent with the existing choice-count behavior: older ChatOpenAI requests can include the SDK default n=1, but this assignment reports it unconditionally. The OpenAI instrumentation intentionally omits gen_ai.request.choice.count for n=1 (openai/utils.py:137-140, test_chat_completions.py:461-485), so assign only a non-default integer count here and add a regression test for the default.

            request_choice_count = params.get("n")

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Lite

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