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

# Examples

```
# use tonic::metadata::*;
let mut map = MetadataMap::new();
assert!(map.get_bin("trace-proto-bin").is_none());

map.insert_bin("trace-proto-bin", MetadataValue::from_bytes(b"hello"));
assert_eq!(map.get_bin("trace-proto-bin").unwrap(), &"hello");
assert_eq!(map.get_bin("trace-proto-bin").unwrap(), &"hello");

map.append_bin("trace-proto-bin", MetadataValue::from_bytes(b"world"));
assert_eq!(map.get_bin("trace-proto-bin").unwrap(), &"hello");

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

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