perf(span-processor): skip span_formatter when debug logging is off

[DavidTraina]

What does this PR do?

Fixes langfuse/langfuse#15339

LangfuseSpanProcessor.on_end built its debug message with an f-string:

langfuse_logger.debug(
    f"Trace: Processing span name='{span._name}' | Full details:\n{span_formatter(span)}"
)

f-strings are evaluated before debug() is called, so span_formatter(span) -- a json.dumps(indent=2) of the span's attributes, events, links, resource and scope -- ran on every span end even though the langfuse logger defaults to WARNING and the string was then discarded. on_end runs on the thread that ends the span, the event loop in async apps, so this was a per-span CPU cost proportional to span throughput that no log-level setting could switch off.

Measured locally, span_formatter costs 28 us on a small span and 167 us on a 100 KB one; the linked issue reports ~0.27 ms/span. The isEnabledFor check costs 0.02 us.

This guards the call so the serialization only runs when the record would be emitted, and passes the message as lazy %-style args. Output at DEBUG is unchanged.

Adds a parametrized unit test that drives a real span through the processor and asserts span_formatter is not called at WARNING and is called once at DEBUG. The WARNING case fails on main.

Related: #1846 converts the SDK's remaining f-string log calls to lazy formatting and enables ruff G004. It touches this same call to make it lazy but does not add the guard, so whichever merges second needs a trivial rebase here.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor
• Documentation update
• Tooling, CI, or repo maintenance

Verification

uv run --frozen ruff check .                                        # All checks passed!
uv run --frozen ruff format --check .                              # no drift in changed files
uv run --frozen mypy langfuse --no-error-summary                    # exit 0
uv run --frozen pytest tests/unit/test_span_processor.py            # 4 passed
uv run --frozen pytest -n auto --dist worksteal tests/unit          # 688 passed, 2 skipped

e2e / live_provider were not run: unit-testable change, no network behavior touched.

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; if generated files changed, I used the upstream regeneration path.
• I did not commit secrets or credentials.

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