Returns preprocessor for incl combined features in StandardMetricInputs.

feature_keys List of feature keys. An empty list means all.
include_default_inputs True to include default inputs (labels, predictions, example weights) in addition to the transformed features.
model_names Optional model names (required if transformed_features used with multi-model evaluations).
output_names Optional output names. Only used if include_default_inputs is True. If unset all outputs will be included with the default inputs.