feat: add _on_ending hook support to span processors

- Implemented _on_ending method in AISpanProcessor and BraintrustSpanProcessor to forward pre-end hooks.
- Added a helper function _forward_on_ending to handle compatibility with different OpenTelemetry SDK versions.
- Updated tests to verify the presence and functionality of the new _on_ending method.

Author: colinbennettbrain

py/src/braintrust/otel/__init__.py

@@ -43,6 +43,18 @@ def get_tracer_provider():
 FILTER_PREFIXES = ("gen_ai.", "braintrust.", "llm.", "ai.", "traceloop.")
 
 
+def _forward_on_ending(processor, span) -> None:
+    """
+    Forward OpenTelemetry's optional _on_ending hook when available.
+
+    Newer OpenTelemetry SDK versions call _on_ending before on_end. Older SDK
+    versions may not implement the hook on wrapped processors.
+    """
+    on_ending = getattr(processor, "_on_ending", None)
+    if callable(on_ending):
+        on_ending(span)
+
+
 class AISpanProcessor:
     """
     A span processor that filters spans to only export filtered telemetry.
@@ -79,6 +91,13 @@ def on_end(self, span):
         if self._should_keep_filtered_span(span):
             self._processor.on_end(span)
 
+    def _on_ending(self, span):
+        """
+        Forward pre-end hook for kept spans when the wrapped processor supports it.
+        """
+        if self._should_keep_filtered_span(span):
+            _forward_on_ending(self._processor, span)
+
     def shutdown(self):
         """Shutdown the inner processor."""
         self._processor.shutdown()
@@ -322,6 +341,10 @@ def on_end(self, span):
         """Forward span end events to the inner processor."""
         self._processor.on_end(span)
 
+    def _on_ending(self, span):
+        """Forward pre-end hook when the wrapped processor supports it."""
+        _forward_on_ending(self._processor, span)
+
     def shutdown(self):
         """Shutdown the inner processor."""
         self._processor.shutdown()

py/src/braintrust/test_otel.py

@@ -191,7 +191,9 @@ def test_braintrust_otel_filter_ai_spans_environment_variable():
         # Verify it has the expected attributes
         assert hasattr(filter_processor, "_processor")
         assert hasattr(filter_processor, "_custom_filter")
+        assert hasattr(filter_processor, "_on_ending")
         assert hasattr(filter_processor, "_should_keep_filtered_span")
+        assert callable(filter_processor._on_ending)
         assert callable(filter_processor._should_keep_filtered_span)
 
     finally:
@@ -216,10 +218,12 @@ def test_braintrust_span_processor_class():
         # Should have the span processor interface
         assert hasattr(processor, "on_start")
         assert hasattr(processor, "on_end")
+        assert hasattr(processor, "_on_ending")
         assert hasattr(processor, "shutdown")
         assert hasattr(processor, "force_flush")
         assert callable(processor.on_start)
         assert callable(processor.on_end)
+        assert callable(processor._on_ending)
         assert callable(processor.shutdown)
         assert callable(processor.force_flush)
 
@@ -235,6 +239,7 @@ def test_braintrust_span_processor_class():
         # Should have the same interface
         assert hasattr(processor_with_filtering, "on_start")
         assert hasattr(processor_with_filtering, "on_end")
+        assert hasattr(processor_with_filtering, "_on_ending")
         assert hasattr(processor_with_filtering, "shutdown")
         assert hasattr(processor_with_filtering, "force_flush")