Adding star-tree related stats documentation. (opensearch-project#10667)

* Adding star-tree related stats description.

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

* Apply suggestions from code review

Signed-off-by: kolchfa-aws <[email protected]>

---------

Signed-off-by: Sandesh Kumar <[email protected]>
Signed-off-by: kolchfa-aws <[email protected]>
Co-authored-by: kolchfa-aws <[email protected]>

Author: 2 people

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

@@ -877,6 +877,9 @@ search.concurrent_avg_slice_count	| Integer | The average slice count of all sea
 search.concurrent_query_total	|Integer | The total number of query operations that use concurrent segment search.	
 search.concurrent_query_time_in_millis	| Integer | The total amount of time taken by all query operations that use concurrent segment search, in milliseconds.	
 search.concurrent_query_current	|Integer | The number of currently running query operations that use concurrent segment search.	
+search.startree_query_total | Integer | The total number of query operations that use a star tree for search.	
+search.startree_query_time_in_millis | Integer | The total amount of time taken by all query operations that use a star tree for search, in milliseconds.	
+search.startree_query_current | Integer | The number of currently running query operations that use a star tree for search.	
 search.open_contexts | Integer | The number of open search contexts.
 search.query_total | Integer | The total number of shard query operations.
 search.query_time_in_millis | Integer | The total amount of time for all shard query operations, in milliseconds.