Computes the z-score for each series within a group-by family. For each group at each timestamp, the output is
(value - mean) / stddev, where mean and stddev are computed across all groups in the family at that timestamp,
using the interpolation configuration to align values across groups. When stddev is zero at a timestamp (e.g., all groups have the same value), that data point is excluded from the output.
If groupByTags is empty, all groups in the input series will be considered as a single family. This node only
produces meaningful results when the input has a group by.