Function begin_cloud_invoke_tracer 

pub async fn begin_cloud_invoke_tracer(
    trace: &TraceContext,
    provider: CloudProviderId,
    cloud_model: &str,
    messages: &[SharedMessage],
    system_prompt: Option<&str>,
) -> Option<RequestTracer>

Begin tracing one explicit cloud invocation (AWS Bedrock, Azure OpenAI, Google Vertex). Always TransportKind::CloudProxy with wire: None: the provider body is serialized and signed behind the broker boundary, so this process never holds the exact bytes sent. Returns None when tracing is disabled or identity is unrepresentable.