Add new sampling method to the downsample API

specification/indices/_types/Downsample.ts

@@ -24,4 +24,13 @@ export class DownsampleConfig {
    * The interval at which to aggregate the original time series index.
    */
   fixed_interval: DurationLarge
+  /**
+   * The sampling method used to reduce the documents; it can be either `aggregate` or `last_value`. Defaults to `aggregate`.
+   */
+  sampling_method?: SamplingMethod
+}
+
+enum SamplingMethod {
+  aggregate,
+  last_value
 }

specification/indices/_types/DownsamplingRound.ts

@@ -17,16 +17,15 @@
  * under the License.
  */
 
-import { Duration } from '@_types/Time'
-import { DownsampleConfig } from '@indices/_types/Downsample'
+import { Duration, DurationLarge } from '@_types/Time'
 
 export class DownsamplingRound {
   /**
    * The duration since rollover when this downsampling round should execute
    */
   after: Duration
   /**
-   * The downsample configuration to execute.
+   * The downsample interval.
    */
-  config: DownsampleConfig
+  fixed_interval: DurationLarge
 }

specification/indices/downsample/Request.ts

@@ -23,13 +23,18 @@ import { DownsampleConfig } from '@indices/_types/Downsample'
 
 /**
  * Downsample an index.
- * Aggregate a time series (TSDS) index and store pre-computed statistical summaries (`min`, `max`, `sum`, `value_count` and `avg`) for each metric field grouped by a configured time interval.
+ * Downsamples a time series (TSDS) index and reduces its size by keeping the last value or by pre-aggregating metrics:
+ *
+ * - When running in `aggregate` mode, it pre-calculates and stores statistical summaries (`min`, `max`, `sum`, `value_count` and `avg`)
+ * for each metric field grouped by a configured time interval and their dimensions.
+ * - When running in `last_value` mode, it keeps the last value for each metric in the configured interval and their dimensions.
+ *
  * For example, a TSDS index that contains metrics sampled every 10 seconds can be downsampled to an hourly index.
  * All documents within an hour interval are summarized and stored as a single document in the downsample index.
  *
  * NOTE: Only indices in a time series data stream are supported.
  * Neither field nor document level security can be defined on the source index.
- * The source index must be read only (`index.blocks.write: true`).
+ * The source index must be read-only (`index.blocks.write: true`).
  * @doc_id indices-downsample-data-stream
  * @rest_spec_name indices.downsample
  * @availability stack since=8.5.0 stability=experimental