docs: public Collection<String> getRequestVariables() {
      final Collection<String> variables = new LinkedHashSet<>(this.uriTemplate.getVariables());
      this.queries.values().forEach(queryTemplate -> variables.addAll(queryTemplate.getVariables()));
      this.headers.values()
          .forEach(headerTemplate -> variables.addAll(headerTemplate.getVariables()));
      return variables;
    }