HLL field Mapper for Cardinality Rollups

[sandeshkr419]

Description

The hll field type stores pre-aggregated cardinality data using HyperLogLog++ (HLL++) sketch data structures. This field type is designed for internal use by OpenSearch and its plugins (such as the ISM plugin for multi-tier rollup operations) to enable efficient cardinality aggregation on rolled-up data.

Key Features

• Efficient Storage: Stores approximate cardinality estimates in compact binary format sketches
• Merge Support: Supports merging multiple HLL++ sketches during aggregation
• Configurable Precision: Allows tuning the accuracy-memory tradeoff via precision parameter
• Rollup Integration: Designed for use with ISM plugin rollup operations

Parameters

• precision (optional): Integer between 4 and 18 (consistent with OpenSearch) that controls the accuracy-memory tradeoff
  • Default: 12 (keeping consistent with OpenSearch Cardinality Aggregation)
  • Higher precision = better accuracy but more memory

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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✨ Finishing touches

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❌ Gradle check result for 97b7392: FAILURE

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