MergeIntoSpec:
+ https://github.com/apache/iceberg/issues/7431 
A merge-on-read table
- should create no new files for merge-on-read
  + Given tables polaris.my_namespace.MergeIntoSpec and polaris.my_namespace.OtherTable both created as 'merge-on-read' containing data that is the same other than the labels column 
  + When we merge them with:

MERGE INTO polaris.my_namespace.MergeIntoSpec t USING (
  SELECT
    *
  FROM
    polaris.my_namespace.OtherTable
) s ON s.id = t.id
WHEN MATCHED THEN
UPDATE
SET
  t.label = s.label
  WHEN NOT MATCHED THEN INSERT * 
  + Then the target table polaris.my_namespace.MergeIntoSpec has the labels from the source table polaris.my_namespace.OtherTable 
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