ValueMappingEstimator does not produce the correct output schema with metadata

[singlis]

The ValueMappingEstimator when calling GetOutputSchema and setting the treatAsKeyTypes to true does not return an output schema that contains metadata. This is can be reproduced by creating a ValueMappingEstimator and appending with a KeyToValueMappingEstimator to do a reverse lookup of the KeyType to the original value.

Here is the code snippet that reproduces the error:

            var pipeline = new ValueMappingEstimator<string, string>(ml, educationKeys, educationValues, true, ("Education", "EducationKeyType"))
                              .Append(new KeyToValueMappingEstimator(ml, ("EducationKeyType", "EducationCategory")));