I'm trying to fill a Grid with Data from a Stream. I want one of the columns of the grid to be the "timestamp" field from the stream. However, when I "Configure Grid Columns" the "timestamp" field is not available as one of the columns to configure. The only columns that I can see are the ones that belong to the Data Shape that my Stream is based on. The default fields of a Stream do not seem to be available. How I can enable them?