Function probe_once 

pub async fn probe_once(
    kind: ProbeKind,
    tls: Option<&Arc<ClientConfig>>,
    address: &str,
    port: u16,
    path: &str,
    timeout: Duration,
    happy: Option<Duration>,
) -> bool

One direct probe of address:port, bounded by timeout (connect plus response for http). tls is the upstream’s outbound TLS client config: Some for https/http2 upstreams (built from the SAME root store as the pooled connector — webpki by default, the upstream’s trusted_ca_file bundle when configured, #121 — with HTTP/1.1 ALPN), None for plaintext http1 upstreams. happy is the upstream’s resolved happy-eyeballs delay (DW-030): probes dial through the same RFC 8305 racing as the pooled connector, so a dual-stack endpoint’s probe does not silently behave differently from its traffic. Returns whether the endpoint answered healthy.