pub fn append_bin<K>(&mut self, key: K, value: MetadataValue<Binary>) -> bool
    where
        K: IntoMetadataKey<Binary>,
    {
        key.append(self, value)
    }
Like append, but for binary keys (for example "trace-proto-bin").

This method panics when the given key is a string and it cannot be
converted to a `MetadataKey<Binary>`.

# Examples

```
# use tonic::metadata::*;
let mut map = MetadataMap::new();
assert!(map.insert_bin("trace-proto-bin", MetadataValue::from_bytes(b"world")).is_none());
assert!(!map.is_empty());

map.append_bin("trace-proto-bin", MetadataValue::from_bytes(b"earth"));

let values = map.get_all_bin("trace-proto-bin");
let mut i = values.iter();
assert_eq!("world", *i.next().unwrap());
assert_eq!("earth", *i.next().unwrap());
```

```should_panic
# use tonic::metadata::*;
let mut map = MetadataMap::new();
// Trying to append a key that is not valid panics.
map.append_bin("x{}host-bin", MetadataValue::from_bytes(b"world")); // This line panics!
```

```should_panic
# use tonic::metadata::*;
let mut map = MetadataMap::new();
// Trying to append a key that is ascii panics (use append).
map.append_bin("x-host", MetadataValue::from_bytes(b"world")); // This line panics!
```
tonic/src/metadata/map.rs