Added documentation for admission control stats

Signed-off-by: Ajay Kumar Movva <[email protected]>

_api-reference/nodes-apis/nodes-stats.md

@@ -51,6 +51,7 @@ adaptive_selection | Statistics about adaptive replica selection, which selects
 script_cache | Statistics about script cache.
 indexing_pressure | Statistics about the node's indexing pressure.
 shard_indexing_pressure | Statistics about shard indexing pressure.
+admission_control | Statistics about admission control.
 
 To filter the information returned for the `indices` metric, you can use specific `index_metric` values. You can use these only when you use the following query types:
 
@@ -708,6 +709,16 @@ Select the arrow to view the example response.
         },
         "enabled" : false,
         "enforced" : false
+      },
+      "admission_control": {
+        "global_cpu_usage": {
+          "transport": {
+            "rejection_count": {
+              "search": 3,
+              "indexing": 1
+            }
+          }
+        }
       }
     }
   }
@@ -761,6 +772,7 @@ http.total_opened | Integer | The total number of HTTP connections the node has
 [indexing_pressure](#indexing_pressure) | Object | Statistics related to the node's indexing pressure.
 [shard_indexing_pressure](#shard_indexing_pressure) | Object | Statistics related to indexing pressure at the shard level.
 [search_backpressure]({{site.url}}{{site.baseurl}}/opensearch/search-backpressure#search-backpressure-stats-api) | Object | Statistics related to search backpressure.
+[admission_control](#admission_control) | Object | Statistics related to admission control for the node.
 
 ### `indices`
 
@@ -1192,6 +1204,15 @@ total_rejections_breakup_shadow_mode.throughput_degradation_limits | Integer | T
 enabled | Boolean | Specifies whether the shard indexing pressure feature is turned on for the node.
 enforced | Boolean | If true, the shard indexing pressure runs in enforced mode (there are rejections). If false, the shard indexing pressure runs in shadow mode (there are no rejections, but statistics are recorded and can be retrieved in the `total_rejections_breakup_shadow_mode` object). Only applicable if shard indexing pressure is enabled. 
 
+### `admission_control`
+
+The `admission_control` object contains the rejection count of search and indexing requests based on resource consumption and has the following properties.
+Field | Field type | Description
+:--- | :--- | :---
+admission_control.global_cpu_usage.transport.rejection_count.search | Integer | The total number of search rejections in the transport layer when the node CPU usage threshold limit was breached. In this case, additional search requests are rejected until the system recovers.
+admission_control.global_cpu_usage.transport.rejection_count.indexing | Integer | The total number of indexing rejections in the transport layer when the node CPU usage threshold limit was breached. In this case, additional indexing requests are rejected until the system recovers.
+
+
 ## Concurrent segment search
 
 Starting in OpenSearch 2.10, [concurrent segment search]({{site.url}}{{site.baseurl}}/search-plugins/concurrent-segment-search/) allows each shard-level request to search segments in parallel during the query phase. If you [enable the experimental concurrent segment search feature flag]({{site.url}}{{site.baseurl}}/search-plugins/concurrent-segment-search#enabling-the-feature-flag), the Nodes Stats API response will contain several additional fields with statistics about slices (units of work executed by a thread). For the descriptions of those fields, see [Index Stats API]({{site.url}}{{site.baseurl}}/api-reference/index-apis/stats#concurrent-segment-search).