NGINX log filter $upstream_response_time JSON ELK "-" parsefailure
Try this:
map $upstream_response_time $temprt { default $upstream_response_time; "" 0;}$upstream_response_time either a number or unset. Nginx logs unset variables as dash (-), but map treats them as empty strings.
A fleshed out example from @AlexeyTen using null instead of 0 to differentiate from real values since I wasn't 100% sure how to use mapped variables:
map $upstream_response_time $temprt { default $upstream_response_time; "" null;}log_format le_json '{ "@timestamp": "$time_iso8601", ' '"remote_addr": "$remote_addr", ' '"remote_user": "$remote_user", ' '"body_bytes_sent": "$body_bytes_sent", ' '"status": $status, ' '"request": "$request", ' '"request_method": "$request_method", ' '"response_time": $temprt, ' '"http_referrer": "$http_referer", ' '"http_user_agent": "$http_user_agent" }';