Update http_requests.py

opensearchpy/connection/http_requests.py

@@ -216,7 +216,7 @@ def perform_request(  # type: ignore
 
         # Add the service time to the response headers
         if calculate_service_time:
-            response.headers["X-Service-Time"] = str(duration)
+            raw_data["__client"] = {"Service_Time": str(duration)}
 
         # raise warnings if any from the 'Warnings' header.
         warnings_headers = (