How to use MEF Inherited Export & MetaData?

I have an interface:

[InheritedExport(typeof(IMetric))]
public interface IMetric { ... }

I have a Meta attribute interface:

 public interface IMetricAttribute { ... }

and an attribute that implements it:

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
public class MetricAttribute : ExportAttribute, IMetricAttribute {
    public string MetricName { get; set; }
    public string MetricDescription { get; set; }

    public MetricAttribute(string name, string description)
        : base(typeof(MetricAttribute)) {
        this.MetricName = name;
        this.MetricDescription = description;
    }
}

I then have two classes:

[Metric("MetricA","MetricA")]
public class MetricA: IMetric { ... }

[Export(typeof(IMetric))] <<<< THIS IS IMPORTANT
[Metric("MetricB", "MetricB")]
public class MetricB: IMetric { ... }

I then try to import the metrics ( i can see both in the cataloge)

The following returns be MetricA AND MetricB

var metrics = compositionContainer.GetExports<IMetric>();

However the following returns ONLY MetricB and NOT MetricA

var metrics = compositionContainer.GetExports<IMetric, IMetricAttribute>();

any idea why?

(note the duplicate export on MetricB (it already has it from implementing IMetric))

thanks

David