@Nullable
  public String getKey() {
    return key;
  }
Returns the filter key of the tag. Tags with the same key are
mutually exclusive; a spec element cannot have two tags with
the same key. A tag on a child element overrides any tag with
the same key on a parent element.

The filter key may be {@code null}, in which case the tag
will not be amenable to filtering.

@return the filter key of the tag
spock-core/src/main/java/org/spockframework/runtime/model/Tag.java