code: protected String getValue(Trace trace, Node node, Direction direction,
              Map<String, ?> headers, Object[] values) {
          if (expression != null) {
              return expression.evaluate(trace, node, direction, headers, values);
          }
  
          return null;
      }
docs: This method returns the value, associated with the expression, for the
  supplied data.
  
  @param trace The trace
  @param node The node
  @param direction The direction
  @param headers The optional headers
  @param values The values
  @return The result of the expression