Another example how to use log_append for reverse proxy upstream details

[steffenbusch]

I have added one of my use-cases for log_append as another example. When using multiple reverse_proxy upstreams, it can be useful to have the upstream as well as time and latency in the logs using the placeholders from https://caddyserver.com/docs/json/apps/http/servers/routes/handle/reverse_proxy/#docs