Function serve_listener 

pub(crate) async fn serve_listener(
    name: &str,
    addr: &str,
    router: Router,
    tls: Option<TlsConfig>,
)

Serve an axum router on addr, using TLS when tls is Some and the binary was built with --features tls. Falls back to plaintext otherwise (warning if TLS was requested but unavailable). Used for the metrics and admin management listeners; the OCSP data plane is always plaintext.