Using Label.Process and Label.Json in Grafana River Flow Unable to add custom labels to json

I am parsing an nginx log that outputting JSON and using river flow to process these.
I want to dynamically add the label of [status] ( which is the nginx status code 200, 204 etc ) which exists in the log entry.
There are NO errors in the grafana log; no errors in the monitoring UI and all dependencies are there as expected
But the logs are never labelled with status; only “app” and “instance” are there?

My river config below is

local.file_match "nginx2" {
	path_targets = [
		{__path__ = "/var/log/nginx/*.json", "instance" = "instance1", "app" = "myapp" }]
}

loki.source.file "nginx2" {
	targets    = local.file_match.nginx2.targets
	forward_to = [loki.process.nginx2.receiver]
}

loki.process "nginx2" {
  forward_to = [loki.write.endpoint.receiver]
 stage.json {
      // extract the status from the json to local map 
      expressions = {"status" = "status" }
  }

  stage.labels {
       // label the log row with the status which should appear as "mystatus"
	values = {  "mystatus" = "status" }
  }

loki.write "endpoint" {
	endpoint {
		url = "http://myloki:3100/loki/api/v1/push"
	}
}
}

Don’t see anything obvious wrong. Do you have an example logline?

Thanks for your reply…

[ solved ] The logline was wrong it had a trailing comma.
The Grafana UI could convert to json via | json and everything looked find but the river agent didnt like it. Nor did it throw any warnings or errors.