Macro log_warn 

macro_rules! log_warn {
    ($ctx:expr, $msg:expr) => { ... };
    ($ctx:expr, $msg:expr, $($key:ident => $value:expr),+ $(,)?) => { ... };
    ($ctx:expr, $fmt:expr, $($arg:tt)*) => { ... };
}

Logs a message at the WARN level with request context.