pub fn get_mut<K>(&mut self, key: K) -> Option<&mut MetadataValue<Ascii>>
    where
        K: AsMetadataKey<Ascii>,
    {
        key.get_mut(self)
    }
Returns a mutable reference to the value associated with the key. This
method is for ascii metadata entries (those whose names don't end with
"-bin"). For binary entries, use get_mut_bin.

If there are multiple values associated with the key, then the first one
is returned. Use `entry` to get all values associated with a given
key. Returns `None` if there are no values associated with the key.

# Examples

```
# use tonic::metadata::*;
let mut map = MetadataMap::default();
map.insert("x-host", "hello".parse().unwrap());
map.get_mut("x-host").unwrap().set_sensitive(true);

assert!(map.get("x-host").unwrap().is_sensitive());

// Attempting to read a key of the wrong type fails by not
// finding anything.
map.append_bin("host-bin", MetadataValue::from_bytes(b"world"));
assert!(map.get_mut("host-bin").is_none());
assert!(map.get_mut("host-bin".to_string()).is_none());
assert!(map.get_mut(&("host-bin".to_string())).is_none());

// Attempting to read an invalid key string fails by not
// finding anything.
assert!(map.get_mut("host{}").is_none());
assert!(map.get_mut("host{}".to_string()).is_none());
assert!(map.get_mut(&("host{}".to_string())).is_none());
```
tonic/src/metadata/map.rs