Add new sampling method to the downsample API

[gmarouli]

In elastic/elasticsearch#136633 we introduce one more configuration option in the downsample API that allows a user to choose the sampling method between:

• aggregate: when supported, this will downsample a metric by summarising its values in a aggregate document. This remains the default
• last_value: this will keep the last value and discard the rest.

POST /my-time-series-index/_downsample/my-downsampled-time-series-index
{
  "fixed_interval": "1d",
  "sampling_method": "last_value"
}

Waiting for to be merged.

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[pquentin]

Thanks! LGTM. We won't have YAML test validation until elastic/elasticsearch#137308.

[gmarouli]

Replaced by #5563