fix(otel): keep last duplicate span in masking batches

[hassiebp]

What changed

• keep the last span when a mask_otel_spans export batch contains duplicate (trace_id, span_id) identifiers
• preserve unrelated spans and apply masking patches to the retained duplicate
• emit one warning with duplicate and remaining-span counts
• document the keep-last masking contract
• add exporter-local regression coverage

Why

The masking hook exposes each batch as a mapping keyed by trace and span ID. Previously, one duplicate identifier caused the SDK to report export success without calling the underlying exporter, silently dropping the entire batch. Replay-safe deterministic instrumentation such as Temporal workflows can legitimately produce this condition.

Linear: LFE-14818

Validation

• uv run --frozen pytest -q tests/unit/test_mask_otel_spans.py — 51 passed
• uv run --frozen ruff check .
• uv run --frozen ruff format --check .
• uv run --frozen mypy langfuse --no-error-summary
• commit pre-commit hooks: ruff check, ruff format, mypy

Greptile Summary

The PR changes OpenTelemetry masking batches to retain the last span for each duplicate trace/span identifier instead of dropping the entire batch.

• Deduplicates valid spans before invoking the masking callback.
• Preserves unrelated spans and applies masking patches to retained duplicates.
• Emits one aggregate warning with duplicate and remaining-span counts.
• Documents and tests the keep-last callback contract.

Confidence Score: 5/5

The PR appears safe to merge because no concrete blocking or independently actionable non-blocking issue was identified.

The new path deterministically retains the final valid span for each duplicate identifier, preserves unrelated spans, applies patches to the retained span, and exports the resulting batch as documented.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[OTel export batch] --> B[Process span attributes and media]
  B --> C[Discard spans with invalid context]
  C --> D[Key spans by trace ID and span ID]
  D --> E{Duplicate identifier?}
  E -->|Yes| F[Replace prior entry with last span]
  E -->|No| G[Retain span]
  F --> H[Emit aggregate warning]
  G --> I[Build masking callback parameters]
  H --> I
  I --> J[Apply returned patches]
  J --> K[Clone retained spans]
  K --> L[Underlying exporter]

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_{Reviews (1): Last reviewed commit: "fix(otel): keep last duplicate span in m..." | Re-trigger Greptile}

Context used:

• Knowledge Base — OTel Span Processing and Export Pipeline
• Knowledge Base — Utils and Support Types

[github-actions]

@claude review