Function shutdown 

pub(crate) fn shutdown(
    exit_code: i32,
    sentinel: Option<&SentinelClient>,
    run_ctx: Option<Arc<Mutex<RunContext>>>,
    shutdown_flag: Option<Arc<AtomicBool>>,
    upload_handle: Option<JoinHandle<Vec<String>>>,
    remaining: Vec<Sample>,
    interval_secs: u64,
) -> !

Flush remaining samples, close the Sentinel run, then exit.

Called on both shell-wrapper child exit and SIGTERM. Replaces the former bare std::process::exit() calls so the upload thread always gets a chance to flush.