public void registerMetadataExpression(String expression, int targetDepth, String storageKey) {
        metadataExpressions.add(new MetadataExpression(expression, targetDepth, storageKey));
    }
Registers an expression, which if matched, will cause the data for the
matching element to be stored in the metadata map under the specified
key.

@param expression
           The expression an element must match in order for it's data to
           be pulled out and stored in the metadata map.
@param targetDepth
           The depth in the XML document where the expression match must
           start.
@param storageKey
           The key under which to store the matching element's data.
aws-java-sdk-core/src/main/java/com/amazonaws/transform/StaxUnmarshallerContext.java