As far as I know, Picketlink doesn't generate metadata automatically. It only supports the way around. That is you can configure a SP by using metadata. But you can't export the metadata from a Picketlink SP. if you want to register a picketlink SP to other IdP by using metadata, you have to manually prepare the metadata xml.
Just follow some examples and replace the values in the xml accordingly. Picketlink quickstarts include a SP metadata xml, you can find it here: https://github.com/jboss-developer/jboss-picketlink-quickstarts/blob/master/picketlink-federation-saml-sp-with-metadata/src/main/resources/sp-metadata.xml