[ML][Inference][HLRC] add GET _stats (#49562)

docs/java-rest/high-level/ml/get-trained-models-stats.asciidoc

@@ -0,0 +1,42 @@
+--
+:api: get-trained-models-stats
+:request: GetTrainedModelsStatsRequest
+:response: GetTrainedModelsStatsResponse
+--
+[role="xpack"]
+[id="{upid}-{api}"]
+=== Get Trained Models Stats API
+
+experimental[]
+
+Retrieves one or more Trained Model statistics.
+The API accepts a +{request}+ object and returns a +{response}+.
+
+[id="{upid}-{api}-request"]
+==== Get Trained Models Stats request
+
+A +{request}+ requires either a Trained Model ID, a comma-separated list of
+IDs, or the special wildcard `_all` to get stats for all Trained Models.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-request]
+--------------------------------------------------
+<1> Constructing a new GET request referencing an existing Trained Model
+<2> Set the paging parameters
+<3> Allow empty response if no Trained Models match the provided ID patterns.
+    If false, an error will be thrown if no Trained Models match the
+    ID patterns.
+
+include::../execution.asciidoc[]
+
+[id="{upid}-{api}-response"]
+==== Response
+
+The returned +{response}+ contains the statistics
+for the requested Trained Model.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-response]
+--------------------------------------------------

docs/java-rest/high-level/supported-apis.asciidoc

@@ -302,6 +302,7 @@ The Java High Level REST Client supports the following Machine Learning APIs:
 * <<{upid}-evaluate-data-frame>>
 * <<{upid}-explain-data-frame-analytics>>
 * <<{upid}-get-trained-models>>
+* <<{upid}-get-trained-models-stats>>
 * <<{upid}-delete-trained-model>>
 * <<{upid}-put-filter>>
 * <<{upid}-get-filters>>
@@ -356,6 +357,7 @@ include::ml/stop-data-frame-analytics.asciidoc[]
 include::ml/evaluate-data-frame.asciidoc[]
 include::ml/explain-data-frame-analytics.asciidoc[]
 include::ml/get-trained-models.asciidoc[]
+include::ml/get-trained-models-stats.asciidoc[]
 include::ml/delete-trained-model.asciidoc[]
 include::ml/put-filter.asciidoc[]
 include::ml/get-filters.asciidoc[]