Fix bug with while adding DistinctSketches

[adarshsanjeev]

Fix a bug where we incorrectly call retainedKeys.firstKey() twice while adding another sketch. This can sometimes throw an exception if there was only one key, and/or create an inaccurate estimate of the bytes retained.

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• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
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• a release note entry in the PR description.
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