Function Fn 

pub fn Fn<R: Runtime>(
    ApplicationHandle: &AppHandle<R>,
    EventName: &str,
    Payload: Value,
) -> Result<()>

Emit a Tauri event with a _traceparent field merged into its JSON payload. Payload must be a JSON object (or null - we’ll build one). Non-object payloads pass through unchanged so existing emit sites that send raw arrays / numbers / strings stay correct.

Release builds: cfg!(debug_assertions) short-circuits to a plain app_handle.emit(...) so no traceparent bytes ship to production.