[DOCS] DSL downsampling docs (#103148)

docs/reference/data-streams/lifecycle/apis/put-lifecycle.asciidoc

@@ -59,6 +59,16 @@ duration the document could be deleted. When empty, every document in this data
 If defined, it turns data streqm lifecycle on/off (`true`/`false`) for this data stream.
 A data stream lifecycle that's disabled (`enabled: false`) will have no effect on the
 data stream. Defaults to `true`.
+
+`downsampling`::
+(Optional, array)
+An optional array of downsampling configuration objects, each defining an `after`
+interval representing when the backing index is meant to be downsampled (the time 
+frame is calculated since the index was rolled over, i.e. generation time) and
+a `fixed_interval` representing the downsampling interval (the minimum `fixed_interval`
+value is `5m`). A maximum number of 10 downsampling rounds can be configured.
+See <<data-streams-put-lifecycle-downsampling-example, configuration example>> below.
+
 ====
 
 [[data-streams-put-lifecycle-example]]
@@ -84,3 +94,29 @@ When the lifecycle is successfully updated in all data streams, you receive the
   "acknowledged": true
 }
 --------------------------------------------------
+
+[[data-streams-put-lifecycle-downsampling-example]]
+==== {api-examples-title}
+
+The following example configures two downsampling rounds, the first one starting
+one day after the backing index is rolled over (or later, if the index is still 
+within its write-accepting <<time-bound-indices, time bounds>>) with an interval
+of `10m`, and a second round starting 7 days after rollover at an interval of `1d`:
+
+[source,console]
+--------------------------------------------------------------------
+PUT _data_stream/my-weather-sensor-data-stream/_lifecycle
+{
+    "downsampling": [
+      {
+        "after": "1d",
+        "fixed_interval": "10m"
+      },
+      {
+        "after": "7d",
+        "fixed_interval": "1d"
+      }
+    ]
+}
+--------------------------------------------------------------------
+//TEST[skip:downsampling requires waiting for indices to be out of time bounds]

docs/reference/data-streams/lifecycle/index.asciidoc

@@ -18,6 +18,10 @@ and backwards incompatible mapping changes.
 * Configurable retention, which allows you to configure the time period for which your data is guaranteed to be stored.
 {es} is allowed at a later time to delete data older than this time period.
 
+A data stream lifecycle also supports downsampling the data stream backing indices.
+See <<data-streams-put-lifecycle-downsampling-example, the downsampling example>> for 
+more details.
+
 [discrete]
 [[data-streams-lifecycle-how-it-works]]
 === How does it work?
@@ -35,7 +39,9 @@ into tiers of exponential sizes, merging the long tail of small segments is only
 fraction of the cost of force mergeing to a single segment. The small segments would usually
 hold the most recent data so tail mergeing will focus the merging resources on the higher-value
 data that is most likely to keep being queried.
-4. Applies retention to the remaining backing indices. This means deleting the backing indices whose
+4. If <<data-streams-put-lifecycle-downsampling-example, downsampling>> is configured it will execute 
+all the configured downsampling rounds.
+5. Applies retention to the remaining backing indices. This means deleting the backing indices whose
 `generation_time` is longer than the configured retention period. The `generation_time` is only applicable to rolled over backing
 indices and it is either the time since the backing index got rolled over, or the time optionally configured in the
 <<index-data-stream-lifecycle-origination-date,`index.lifecycle.origination_date`>> setting.