format/prometheus interface missing some metrics

[hejiazhen1203]

format/json will return more metrics than format/prometheus, like upstreamZones.

[kevin-bockman]

@hejiazhen1203 I had a similar problem but it wasn't showing in json either. As long as you have an upstream and there's been activity, it should show. When I was first checking out a Grafana dashboard, I saw the upstream metrics were missing too but I probably hadn't sent any requests to it yet. Once I submitted a request through nginx, the metric started to show up.