mergeHistogram
Merge a remote DDSketch snapshot into this exporter's histogram for key.
Idempotent and lossless: the merged sketch equals the sketch of the combined measurement stream. Returns MetricExportResult.Success after the durable write.
Throws
if remote's relative accuracy is not OTLP-aligned (see alphaForOtlpScale), or if key already holds a sketch with a different configuration (DDSketch configs must match exactly to merge).