fix: openai llm model name is unknown (pipecat-ai#3422)

Author: monster-anshu

changelog/3422.fixed.md

@@ -0,0 +1 @@
+- Fixed a bug in `traced_llm` where the model name in opentelemetry is shown unknown even though it is defined.

src/pipecat/utils/tracing/service_decorators.py

@@ -483,9 +483,10 @@ async def wrapped_start_llm_usage_metrics(tokens):
                             # Add all available attributes to the span
                             attribute_kwargs = {
                                 "service_name": service_class_name,
-                                "model": getattr(
-                                    self, getattr(self, "_full_model_name", "model_name"), "unknown"
-                                ),
+                                "model": getattr(self, "_full_model_name", None)
+                                or getattr(self, "model_name", None)
+                                or params.get("model")
+                                or "unknown",
                                 "stream": True,  # Most LLM services use streaming
                                 "parameters": params,
                             }