pub fn iter(&self) -> ValueIter<'a, VE> {
        ValueIter {
            inner: self.inner.as_ref().map(|inner| inner.iter()),
            phantom: PhantomData,
        }
    }
Returns an iterator visiting all values associated with the entry.

Values are iterated in insertion order.

# Examples

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

let values = map.get_all("x-host");
let mut iter = values.iter();
assert_eq!(&"hello.world", iter.next().unwrap());
assert_eq!(&"hello.earth", iter.next().unwrap());
assert!(iter.next().is_none());
```
tonic/src/metadata/map.rs