feat(otel): add W3C TraceContext propagator for distributed trace correlation

Set up CompositePropagator with TraceContextTextMapPropagator and
W3CBaggagePropagator so that trace context (traceparent/tracestate
headers) is propagated to downstream services via instrumented
HTTP clients (requests, httpx, aiohttp).

Without this, each service starts its own trace root and there is
no way to correlate Open WebUI spans with backend LLM service spans
(e.g. LiteLLM, vLLM) in a trace viewer.

All required classes are already in opentelemetry-api — no new
dependencies needed.

backend/open_webui/utils/telemetry/setup.py

@@ -1,5 +1,9 @@
 from fastapi import FastAPI
 from opentelemetry import trace
+from opentelemetry.propagate import set_global_textmap
+from opentelemetry.propagators.composite import CompositePropagator
+from opentelemetry.trace.propagation.tracecontext import TraceContextTextMapPropagator
+from opentelemetry.baggage.propagation import W3CBaggagePropagator
 
 from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import OTLPSpanExporter
 from opentelemetry.exporter.otlp.proto.http.trace_exporter import (
@@ -30,6 +34,11 @@ def setup(app: FastAPI, db_engine: Engine):
     resource = Resource.create(attributes={SERVICE_NAME: OTEL_SERVICE_NAME})
     if ENABLE_OTEL_TRACES:
         trace.set_tracer_provider(TracerProvider(resource=resource))
+        set_global_textmap(
+            CompositePropagator(
+                [TraceContextTextMapPropagator(), W3CBaggagePropagator()]
+            )
+        )
 
         # Add basic auth header only if both username and password are not empty
         headers = []