fix(langchain): correctly identify inputs for batch lcel chain calls

[sabrenner]

Changes Made

Both batch and invoke calls on LCEL chains for LangChain pass through the same traced method, for code re-use. However, the kwarg for inputs to batch calls is inputs, while for invoke calls is input. This change reflects that, and unit tests have been updated

Motivation

Fix possible breaking customer apps (no reports yet as far as I can tell).

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• 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

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: sabrenner/langchain-chain-inputs-fix
Commit report: 1167ddb
Test service: dd-trace-py

✅ 0 Failed, 128 Passed, 319 Skipped, 5m 28.66s Total duration (1m 40.63s time saved)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 26.57%. Comparing base (dac8aa2) to head (8554905).

Files	Patch %	Lines
ddtrace/contrib/langchain/patch.py	0.00%	8 Missing ⚠️
...ests/contrib/langchain/test_langchain_community.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #9195       +/-   ##
===========================================
- Coverage   78.53%   26.57%   -51.97%     
===========================================
  Files        1277     1262       -15     
  Lines      120384   119955      -429     
===========================================
- Hits        94543    31876    -62667     
- Misses      25841    88079    +62238     

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-05-14 19:17:55

Comparing candidate commit 1167ddb in PR branch sabrenner/langchain-chain-inputs-fix with baseline commit 853d0ef in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 209 metrics, 9 unstable metrics.

[lievan]

Thank you!