public String matchKey() {
      return matchKeys().isEmpty() ? null : matchKeys().getFirst();
    }
Context available after a feature has been matched.
<p>
Adds {@code match_key} and {@code match_value} variables that capture which tag key/value caused the feature to be
included.

@param parent    The parent context
@param matchKeys Keys that triggered the match
planetiler-custommap/src/main/java/com/onthegomap/planetiler/custommap/Contexts.java