Function dial_once 

pub async fn dial_once(
    endpoint: &Handle,
    target: Target,
    to: &Uri,
    options: &DialOptions,
) -> Result<Call>

Place a call, surfacing a 422 instead of retrying it.

dial is this plus one retry, and is what almost everything wants. This is here for a caller that would rather decide for itself what to do about an interval it does not like — a gateway with a policy about how often it is willing to be woken, say.