code: public MigrateArgs<K> key(K key) {
  
          LettuceAssert.notNull(key, "Key must not be null");
  
          this.keys.add(key);
          return this;
      }
docs: Migrate a single {@code key}.
  
  @param key must not be {@literal null}.
  @return {@code this} {@link MigrateArgs}.