Function renewal_due 

pub fn renewal_due(
    setting: KeepWarm,
    cache: CacheUse,
    lead_ms: u64,
) -> Option<u64>

The instant the next renewal request is sent, or None when none is.

lead_ms is the round trip to the provider as the caller measured it: the renewal leaves that long before the cache expires, so the provider reads it while the entry is still alive. A renewal whose instant lies more than one cache lifetime after the last real use is not sent.