docs: public List<String> variables() {
      /* combine the variables from the uri, query, header, and body templates */
      List<String> variables = new ArrayList<>(this.uriTemplate.getVariables());
  
      /* queries */
      for (QueryTemplate queryTemplate : this.queries.values()) {
        variables.addAll(queryTemplate.getVariables());
      }
  
      /* headers */
      for (HeaderTemplate headerTemplate : this.headers.values()) {
        variables.addAll(headerTemplate.getVariables());
      }
  
      /* body */
      variables.addAll(this.body.getVariables());
  
      return variables;
    }