Fix metric in `UncertaintyDownsamplingStrategy`

[MaxiBoether]

Before this, getting the downsampling params of a UncertaintyDownsamplingStrategy failed.

Step 3/n of porting over SIGMOD changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.58%. Comparing base (988bd01) to head (e912659).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #518 +/- ## ========================================== + Coverage 82.50% 82.58% +0.08% ========================================== Files 214 214 Lines 9955 9955 ========================================== + Hits 8213 8221 +8 + Misses 1742 1734 -8 ```

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[github-actions[bot]]

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