pub fn key(&self) -> &MetadataKey<VE> {
        use self::Entry::*;

        MetadataKey::unchecked_from_header_name_ref(match *self {
            Vacant(ref e) => e.inner.key(),
            Occupied(ref e) => e.inner.key(),
        })
    }
Returns a reference to the entry's key

# Examples

```
# use tonic::metadata::*;
let mut map = MetadataMap::new();

assert_eq!(map.entry("x-hello").unwrap().key(), "x-hello");
```
tonic/src/metadata/map.rs