code: public static StreamSpecifier tag(String key, String value) {
      checkValidKey(key);
      checkNotNull(value);
      return new StreamSpecifier("m:" + key + ":" + value);
    }
docs: Matches streams with the metadata tag key having the specified value.
  
  @param key The metadata tag
  @param value The metatdata's value
  @return A new StreamSpecifier