perf(span-processor): defer span_formatter serialization behind DEBUG guard

[samib6]

What does this PR do?

LangfuseSpanProcessor.on_end logs the full span at DEBUG level with an f-string:

langfuse_logger.debug(f"... {span_formatter(span)}")

Because f-strings are built before the call, span_formatter(span) (a full span serialization) runs on every span even when DEBUG logging is off — and logger.debug then discards the string. Since on_end runs once per span, that's wasted work on the hot path when debug isn't enabled. See #15339 for the profiling details.

This guards the log behind isEnabledFor(logging.DEBUG) so the serialization only happens when the output would actually be emitted. I kept the change to this one hot call — the other debug logs in on_end are in rarely-hit early-return/error branches, so I left them alone to keep the diff small.

Fixes langfuse/langfuse#15339

Type of change

• Bug fix

Verification

uv run --frozen pytest tests/unit/test_span_processor.py
uv run --frozen ruff check .
uv run --frozen mypy langfuse --no-error-summary

Added two tests: one checks span_formatter isn't called when the logger is above DEBUG (fails without this change), the other checks it still runs when DEBUG is on.

Checklist

• I self-reviewed the diff using code_review.md.
• I added or updated tests for behavior changes.
• I updated docs, examples, or .env.template if needed. — n/a, internal-only change
• I did not hand-edit generated files.
• I did not commit secrets or credentials.

Greptile Summary

This PR avoids unnecessary span serialization when DEBUG logging is disabled.

• Guards the hot-path debug log with isEnabledFor(logging.DEBUG).
• Adds tests confirming formatting is skipped above DEBUG and retained at DEBUG.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The new guard uses the standard logger’s effective-level check and preserves formatting whenever DEBUG is enabled while removing unnecessary serialization otherwise.

_{Reviews (1): Last reviewed commit: "perf(span-processor): defer span_formatt..." | Re-trigger Greptile}

[claude]

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