pub fn insert_mult(&mut self, value: MetadataValue<VE>) -> ValueDrain<'_, VE> {
        ValueDrain {
            inner: self.inner.insert_mult(value.inner),
            phantom: PhantomData,
        }
    }
Sets the value of the entry.

This function does the same as `insert` except it returns an iterator
that yields all values previously associated with the key.

# Examples

```
# use tonic::metadata::*;
let mut map = MetadataMap::new();
map.insert("host", "world".parse().unwrap());
map.append("host", "world2".parse().unwrap());

if let Entry::Occupied(mut e) = map.entry("host").unwrap() {
let mut prev = e.insert_mult("earth".parse().unwrap());
assert_eq!("world", prev.next().unwrap());
assert_eq!("world2", prev.next().unwrap());
assert!(prev.next().is_none());
}

assert_eq!("earth", map.get("host").unwrap());
```
tonic/src/metadata/map.rs