chore(llmobs): better span linking logic for langchain integration

[sabrenner]

refines the beta span linking logic for llmobs langchain spans to reduce memory

removes the _spans mapping of instance IDs to spans. instead, since we really only used this for steps in chains, and the same runnable (e.g. chat model) could be used in different chains, we want to do this mapping by chain instance.

when "recording" an instance we trace, we always do the span --> instance mapping. however, now the instance --> span mapping is only done as a temporary property on chains, as steps outside of chains will be defaulted to their parent span for linkage. this also makes cleanup easier, as we can delete the property and let GC take care of getting rid of the mapping that's unreferenced.

MLOB-2245

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• The change includes tests OR the PR description describes a testing strategy
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[github-actions]

CODEOWNERS have been resolved as:

ddtrace/llmobs/_integrations/langchain.py                               @DataDog/ml-observability

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-19 16:20:34

Comparing candidate commit 22bfb79 in PR branch sabrenner/langchain-span-linking-improvement with baseline commit 0c7fc1b in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 418 metrics, 2 unstable metrics.