code: public static String getTagValue(Id id, String k) {
      Preconditions.checkNotNull(id, "id");
      return getTagValue(id.tags(), k);
    }
docs: Returns the value associated with with a given key or null if no such key is present in the
  set of tags.
  
  @param id
  Identifier with a set of tags to search.
  @param k
  Key to search for.
  @return
  Value for the key or null if the key is not present.