pub fn get_bin_mut<K>(&mut self, key: K) -> Option<&mut MetadataValue<Binary>>
    where
        K: AsMetadataKey<Binary>,
    {
        key.get_mut(self)
    }
Like get_mut, but for Binary keys (for example "trace-proto-bin").

# Examples

```
# use tonic::metadata::*;
let mut map = MetadataMap::default();
map.insert_bin("trace-proto-bin", MetadataValue::from_bytes(b"hello"));
map.get_bin_mut("trace-proto-bin").unwrap().set_sensitive(true);

assert!(map.get_bin("trace-proto-bin").unwrap().is_sensitive());

// Attempting to read a key of the wrong type fails by not
// finding anything.
map.append("host", "world".parse().unwrap());
assert!(map.get_bin_mut("host").is_none());
assert!(map.get_bin_mut("host".to_string()).is_none());
assert!(map.get_bin_mut(&("host".to_string())).is_none());

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