[Downsampling] Add a new downsampling mode keeping a single sample value

[gmarouli]

Description

Currently, when field type gauge is downsampled, the destination index stores certain aggregate metric in an aggregate metric field type.

We would like to provide an alternative downsampling mode that does not calculate the aggregate metrics but it just keeps the last value. This downsampling method exchanges accuracy for performance and results in a downsampled index with the same mapping as the raw data.

• Extend the API to accept and apply the metric sampling method (Add new sampling method to the Downsample API #136813).
• Expose the sampling method in ILM. (Support different downsampling methods through ILM #136951)
• Expose the sampling method in data stream lifecycle (Support choosing the downsampling method in data stream lifecycle #137023)
• Add telemetry.
• Add documentation

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)