Function with_label 

pub fn with_label<K, V, F, Ret>(k: K, v: V, f: F) -> Ret
where
    K: AsRef<[u8]>,
    V: AsRef<[u8]>,
    F: FnOnce() -> Ret,

Set the label for the specified key to the specified value while the given function is running.

All labels are thread-local: setting a label on one thread has no effect on its value on any other thread.