Creates an extractor for extracting labels.

The extractor's PTransform uses the config's ModelSpec.label_key(s) to lookup the associated label values stored as features under the tfma.FEATURES_KEY (and optionally tfma.TRANSFORMED_FEATURES_KEY) in extracts. The resulting values are then added to the extracts under the key tfma.LABELS_KEY.

eval_config Eval config.

Extractor for extracting labels.