Cache/Queue metrics should not be negative

In the cache metrics collection, we have entries like:

```
{ _id: ObjectId("65d60409acf7b3719825797e"),
  error_code: 'PreviousStepStillProcessingError',
  http_status: 500,
  kind: 'split-duckdb-index-010',
  total: -67 }
```

The total should never be lower than 0
