So I've been trying to get this to integrate with another plugin, specifically Player Rankings, and i found that your API fetch methods weren't working because they are Public methods.  For whatever reason, if the API methods are Public, Rust won't let them be called form other plugins.  When I switched them to Private, they worked just fine.

private object FetchPlayTime(string playerid)
{
if (!_timeCache.ContainsKey(playerid)) return null;
if (!_timeStamps.ContainsKey(playerid)) return _timeCache[playerid].PlayTime;
var additional = CurrentTime() - _timeStamps[playerid];
return _timeCache[playerid].PlayTime + additional;
}
private object FetchAfkTime(string playerid)
{
if (_timeCache.ContainsKey(playerid))
return _timeCache[playerid].AfkTime;
return null;
}
private object FetchReferral(string playerid)
{
if (_timeCache.ContainsKey(playerid))
return _timeCache[playerid].Referrals;
return null;
}

I've posted in Player Rankings Help area how to get that plugin to integrate with yours including changes noted above.   FYI.