pub fn insert(&mut self, value: MetadataValue<VE>) -> MetadataValue<VE> {
        let header_value = self.inner.insert(value.inner);
        MetadataValue::unchecked_from_header_value(header_value)
    }
Sets the value of the entry.

All previous values associated with the entry are removed and the first
one is returned. See `insert_mult` for an API that returns all values.

# Examples

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

if let Entry::Occupied(mut e) = map.entry("host").unwrap() {
let mut prev = e.insert("earth".parse().unwrap());
assert_eq!("hello.world", prev);
}

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