Shards need the collection shard key for filtering, eventually the collection UUID (will temporarily be the namespace until that project is complete), and a chunk version for secondary refresh (SERVER-28120). It will look roughly like this

    {
        "_id" : UUID,
        "ns" : “test.foo”,
        "key" : {
                        "a" : 1
        },
        “secondaryRefreshTo”: ChunkVersion
    }

It should be persisted in MetadataLoader::_initCollection, where we query the config server for the necessary config.collection entry.