feat(llmobs): submit langchain similarity search spans

[yahya-mouman]

This PR adds instrumentation to submit langchain vector similarity search spans to LLM Observability. The similarity search spans sent to LLM Observability will contain the following I/O data:

input.value: a text field annotating the search query
output.value: The output document objects for the vector similarity search

Ticket : https://datadoghq.atlassian.net/browse/MLOB-938

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/feat-llmobs-submit-langchain-similarity-search-spans-c5581deda8b6878e.yaml  @DataDog/apm-python
tests/contrib/langchain/cassettes/langchain_community/openai_pinecone_similarity_search_community.yaml  @DataDog/ml-observability
ddtrace/contrib/internal/langchain/patch.py                             @DataDog/ml-observability
ddtrace/llmobs/_integrations/langchain.py                               @DataDog/ml-observability
tests/contrib/langchain/conftest.py                                     @DataDog/ml-observability
tests/contrib/langchain/test_langchain_llmobs.py                        @DataDog/ml-observability
tests/llmobs/_utils.py                                                  @DataDog/ml-observability

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-08-27 17:59:40

Comparing candidate commit 600a8e1 in PR branch yahya/add-langchain-similarity-search-support with baseline commit 9a114a2 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 353 metrics, 47 unstable metrics.

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: yahya/add-langchain-similarity-search-support
Commit report: e78c7fd
Test service: dd-trace-py

✅ 0 Failed, 3219 Passed, 37666 Skipped, 39m 28.81s Total Time

[Yun-Kim]

One small nit but otherwise lgtm. Thanks!