@Nonnull
        @Override
        public <K> Map<Entity, K> getAllWithKey(@Nonnull MetadataKey<K> key) {
            Objects.requireNonNull(key, "key");
            ImmutableMap.Builder<Entity, K> ret = ImmutableMap.builder();
            this.cache.asMap().forEach((uuid, map) -> map.get(key).ifPresent(t -> {
                Entity entity = Bukkit.getEntity(uuid);
                if (entity != null) {
                    ret.put(entity, t);
                }
            }));
            return ret.build();
        }
The Metadata registries provided by helper.

These instances can be accessed through {@link Metadata}.
helper/src/main/java/me/lucko/helper/metadata/StandardMetadataRegistries.java