Macro log_at 

macro_rules! log_at {
    ($logger:expr, $level:expr, $msg:expr $(,)?) => { ... };
    ($logger:expr, $level:expr, $msg:expr, $($key:ident = $value:expr),+ $(,)?) => { ... };
}

Available on crate feature std only.

Emit a log record at the given level.

The first argument is a Logger (by reference or by value). The second is the level. The third is the message string. Any number of key = expr pairs may follow.