Function with_labels 

pub fn with_labels<I, K, V, F, Ret>(i: I, f: F) -> Ret
where
    I: IntoIterator<Item = (K, V)>,
    K: AsRef<[u8]>,
    V: AsRef<[u8]>,
    F: FnOnce() -> Ret,

Set the labels for the specified keys to the specified values while the given function is running.

i is an iterator of key-value pairs.

The effect is the same as repeatedly nesting calls to the singular with_label.