code: func (b *RWSetBuilder) AddToMetadataWriteSet(ns, key string, metadata map[string][]byte) {
  	b.getOrCreateNsPubRwBuilder(ns).
  		metadataWriteMap[key] = mapToMetadataWrite(key, metadata)
  }
docs: // AddToMetadataWriteSet adds a metadata to a key in the write-set
  // A nil/empty-map for 'metadata' parameter indicates the delete of the metadata