Refactor _to_dict for OTLP-compatible JSON serialization

[dnandakumar-nv]

Summary

• Fix OTLP span attribute encoding failures when exporting telemetry (e.g. to LangSmith)
• _to_dict in SpanExporter previously returned raw Python objects (LangChain messages, dicts with nested None values) as input.value_obj/output.value_obj span attributes, which OTLP's protobuf encoder cannot serialize
• Changed _to_dict to always return a JSON string via json.dumps(result, default=str), guaranteeing OTLP compatibility for all input types
• Added 15 new tests covering Pydantic models, lists of messages, nested None values, arbitrary objects, and the always-returns-string invariant

Test plan

• All 39 tests pass in tests/nat/observability/exporter/test_span_exporter.py (24 existing + 15 new)

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• When the PR is ready for review, new or existing tests cover these changes.
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Summary by CodeRabbit

• Bug Fixes

  • Improved span export serialization reliability with consistent JSON format handling and enhanced fallback mechanisms.

• Tests

  • Added comprehensive test coverage for span serialization across various data types and edge cases.

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[coderabbitai]

Walkthrough

The _to_dict method in SpanExporter was refactored to consistently return JSON string serialization instead of dictionary or raw values. The method now serializes data structures to JSON with fallback to string representation for non-serializable objects. Comprehensive unit tests verify all serialization paths produce string outputs.

Changes

Cohort / File(s)	Summary
Production serialization logic src/nat/observability/exporter/span_exporter.py	Changed _to_dict method return type from dict[str, typing.Any] | typing.Any to str. Refactored logic to serialize output to JSON using json.dumps, with fallback to str(data) for non-serializable objects. Added json import.
Serialization test suite tests/nat/observability/exporter/test_span_exporter.py	Added TestToDictSerialization test class with comprehensive test cases covering string inputs, dict serialization, Pydantic model handling, list serialization, nested dictionaries, non-serializable objects, and exception fallback behavior. Verifies result is always a string. Added imports for json and BaseModel from pydantic.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: refactoring the _to_dict method to ensure OTLP-compatible JSON serialization, which is the core purpose of this PR.
Docstring Coverage	✅ Passed	Docstring coverage is 82.61% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@tests/nat/observability/exporter/test_span_exporter.py`:
- Around line 710-715: The test's BrokenModel.model_dump currently accepts
unused kwargs and raises an exception with a message that triggers Ruff TRY003;
rename the parameter from kwargs to _kwargs in BrokenModel.model_dump to mark it
intentionally unused and either remove the exception message or append a `#
noqa: TRY003` to the raise line—update the BrokenModel class (model_dump)
accordingly so Ruff warnings are silenced.

🧹 Nitpick comments (1)

src/nat/observability/exporter/span_exporter.py (1)

326-350: Avoid blind Exception catch or explicitly justify it.

Ruff BLE001 flags this. If the catch-all is intentional for best‑effort serialization, add a # noqa: BLE001 with a short rationale; otherwise, narrow to expected serialization exceptions to avoid masking bugs.

♻️ Minimal fix to silence Ruff while preserving intent

-        except Exception:
-            return str(data)
+        except Exception:  # noqa: BLE001 - best-effort serialization must never fail span export
+            return str(data)

[dnandakumar-nv]

/ok to test 3c89d8d

[bbednarski9]

an improvement but left some questions above to see if we can harden the serialization to be more robust

[bbednarski9]

I'm worries that the current implementation could be unreliable in the following ways:

1. silent failure str(data) could return non serializable json

2. inconsistent normalization for different inputs like lists and nested dicts.

Something like this could harden it a bit:

'''
def _to_json_string(self, typing.Any) -> str:

def _normalize(obj: typing.Any) -> typing.Any:
    # Pydantic models
    if hasattr(obj, "model_dump"):
        dumped = obj.model_dump(exclude_none=True)
        return _normalize(dumped)

    # Dicts: drop None values, normalize values
    if isinstance(obj, dict):
        return {
            k: _normalize(v)
            for k, v in obj.items()
            if v is not None
        }

    # Lists / tuples: normalize each element
    if isinstance(obj, (list, tuple)):
        return [_normalize(item) for item in obj]

    # Anything else: leave as-is; json.dumps(default=str) will sanitize it
    return obj

try:
    normalized = _normalize(data)
    return json.dumps(normalized, default=str)
except Exception:
    # Last-resort: always valid JSON string, but clearly a fallback
    return str(data)

'''

[bbednarski9]

Would a fallback to a json.dumps be more reliable?