[FEATURE] Cardinality aggregations support for Rollup #1493
Description
Is your feature request related to a problem?
The ISM rollup action does not support Cardinality aggregation. This limitation force customers to build customized transformation pipeline, which undermines the goal of an integrated lifecycle system like ISM.
One sample use case is to calculated the unique number of applications a device is running over a period of time based on the logs.
What solution would you like?
The ask is to enable cardinality aggregations (https://docs.opensearch.org/latest/aggregations/metric/cardinality/) as a supported metrics and also store the HLL_Sketch value as part of the aggregation. The HLL Sketch value can be used for future multi-tiered rollup (#1490).