Add duration to top example

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <[email protected]>

Author: alexellis

docs/openfaas-pro/builder.md

@@ -249,10 +249,13 @@ A HTTP client has three tasks to perform:
             "v: 2022-06-23T09:10:13Z exporting to image 5.18s"
         ],
         "image": "ttl.sh/openfaas-image:1h",
-        "status": "success"
+        "status": "success",
+        "duration": 0.843
     }
     ```
 
+The `duration` field is given in seconds and is the total time taken to build the image including pulling base layers, exporting the image, and pushing it to the registry.
+
 There are several examples available of how to call the Function Builder's API via different programming languages: [openfaas-function-builder-api-examples](https://github.com/openfaas/function-builder-examples)
 
 You should be able to translate the example given with curl into any programming language, but if you need additional support, feel free to [reach out to us](https://openfaas.com/support).