Is it possible to return a CustomMetaData in a @HttpGet?

Is it possible to return a CustomMetaData? I'm getting the error "HttpGet methods do not support return type of InsPackages.Wrapper" when I added "customMetadata" codes on my class. Here's my code:
@RestResource(urlMapping='/api/PackageLicense')
global with sharing class InsPackages{
    @HttpGet   
    global static Wrapper doGet(){
        Wrapper wrap = new Wrapper();
        List<PackageLicense> packageList = [SELECT NamespacePrefix, Status, CreatedDate, LastModifiedDate FROM PackageLicense];
        List<Publisher> publisherList = [SELECT Name, MajorVersion, MinorVersion, NamespacePrefix FROM Publisher];
        Metadata.CustomMetadata customMetadata =  new Metadata.CustomMetadata();
        Metadata.CustomMetadataValue customField = new Metadata.CustomMetadataValue();
        for(PackageLicense p : packageList){
            customField.field = 'NamespacePrefix';
            customField.value = p.NamespacePrefix;
            customMetadata.values.add(customField);
        }
        wrap.customMetadata = customMetadata;
        return wrap;
    }   
    global class Wrapper{
        public List<Metadata.CustomMetadataValue> customMetadata {get; set;}
    }
}