A load that succeeds without its metrics scrape now fails the cell with exit
code 4 and records metrics_scraped in cell.json. The summary reports an
event counter that never registered as zero when the cell is an object store
WAL cell whose scrape succeeded and created objects, and as unknown otherwise.
Planned cells that never ran are read through the same path as recorded ones.
Signed-off-by: jeremyhi <fengjiachun@gmail.com>