Function serve 

pub async fn serve(
    listener: TcpListener,
    metrics: Arc<Metrics>,
    shutdown: Receiver<()>,
) -> Result<()>

Serve /metrics over HTTP on the pre-bound listener until shutdown fires. The shutdown receiver is consumed; send () to stop the accept loop.

Per-connection tasks are spawned with tokio::spawn and detached — they are not tracked or awaited during shutdown. In practice scrapes complete in milliseconds, and dropping an in-flight /metrics response is harmless (the scraper will retry on its next interval). This is a deliberate choice: tracking an unbounded JoinSet alongside the accept loop would add complexity without a consumer that cares about the guarantee.