static MetadataSnapshot empty() {
        return new MetadataSnapshot(null, Collections.emptyMap(), Collections.emptyList(),
                Collections.emptySet(), Collections.emptySet(), Collections.emptySet(), null, Collections.emptyMap(), Cluster.empty());
    }
Copies {@code baseSet} and adds all non-existent elements in {@code fillSet} such that {@code predicate} is true.
In other words, all elements of {@code baseSet} will be contained in the result, with additional non-overlapping
elements in {@code fillSet} where the predicate is true.

@param baseSet the base elements for the resulting set
@param fillSet elements to be filled into the resulting set
@param predicate tested against the fill set to determine whether elements should be added to the base set
clients/src/main/java/org/apache/kafka/clients/MetadataSnapshot.java