When using langfuse_otel callback, empty traces were being sent to Langfuse
for requests that didn't result in actual LLM calls (e.g., auth operations,
health checks, failed requests). These traces contained only internal proxy
operations (auth, postgres, proxy_pre_call) with no useful LLM data.
Root cause: LangfuseOtelLogger extends OpenTelemetry, which sets itself as
the proxy's open_telemetry_logger. This caused create_litellm_proxy_request_started_span
to be called for every request, creating a parent span that was sent to Langfuse
even when no LLM call occurred.
Fix: Override create_litellm_proxy_request_started_span in LangfuseOtelLogger
to return None, preventing the creation of empty parent spans. This is consistent
with the existing overrides for async_service_success_hook and async_service_failure_hook
which already prevent service-level logs from being sent to Langfuse.
Fixes: Empty traces in Langfuse v3 when using langfuse_otel callback
