Add yaml test for tsdb index sorting defaults

Author: jordan-powers

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/tsdb/10_settings.yml

@@ -576,3 +576,29 @@ check time_series empty time bound value:
           "@timestamp": "2021-09-26T03:09:52.123456789Z",
           "metricset": "pod"
         }
+
+---
+default sort field:
+  - requires:
+      cluster_features: [ "mapper.provide_index_sort_setting_defaults" ]
+      reason: "testing index sort setting defaults"
+
+  - do:
+      indices.create:
+        index: test_index
+        body:
+          settings:
+            index:
+              mode: time_series
+              routing_path: foo
+              time_series:
+                start_time: 2021-04-28T00:00:00Z
+                end_time: 2021-04-29T00:00:00Z
+
+  - do:
+      indices.get_settings:
+        index: test_index
+        include_defaults: true
+  - match: { .test_index.settings.index.mode: time_series }
+  - match: { .test_index.defaults.index.sort.field: [ "_tsid", "@timestamp" ] }
+  - match: { .test_index.defaults.index.sort.order: [ "asc", "desc" ] }