Introduces 3 new environmental variables:
- `LOG_FORMAT`
- `LOG_FORMAT_ESCAPE`
- `LOG_JSON`
`LOG_FORMAT` and `LOG_FORMAT_ESCAPE` default to standard values.
When `LOG_JSON` is set, defaults are changed to: `{"time_local":"$time_iso8601","client_ip":"$http_x_forwarded_for","remote_addr":"$remote_addr","request":"$request","status":"$status","body_bytes_sent":"$body_bytes_sent","request_time":"$request_time","upstream_response_time":"$upstream_response_time","upstream_addr":"$upstream_addr","http_referrer":"$http_referer","http_user_agent":"$http_user_agent","request_id":"$request_id"}`and `json`.
See `nginx.tmpl` and https://nginx.org/en/docs/http/ngx_http_log_module.html#log_format for details
