Add missing metrics import

datasets/flwr_datasets/__init__.py

@@ -15,7 +15,7 @@
 """Flower Datasets main package."""
 
 
-from flwr_datasets import partitioner, preprocessor
+from flwr_datasets import metrics, partitioner, preprocessor
 from flwr_datasets import utils as utils
 from flwr_datasets import visualization
 from flwr_datasets.common.version import package_version as _package_version