[feature request] kubeapi-load-balancer's nginx can configure json formatted logs via charm

[Chuan Li]

One of UA customer request a feature that kubeapi-load-balancer charm can configure nginx access_log as json format.

The customer make a general propose as below:
name: nginx_log_format
description: Adds a 'log_format' with the contents of the config value. The log_format will be named 'json_config' and applied to the loadbalancer site's 'server' stanza.
example value: '{'\n '"time_local":"$time_local",'\n '"remote_addr":"$remote_addr",'\n '"remote_user":"$remote_user",'\n '"request":"$request",'\n '"status": "$status",'\n '"body_bytes_sent":"$body_bytes_sent",'\n '"request_time":"$request_time",'\n '"http_referrer":"$http_referer",'\n '"http_user_agent":"$http_user_agent"'\n '}';

example changes this would implement in nginx:
in /etc/nginx/nginx.conf:
+ log_format json_combined escape=json ${value}

in /etc/nginx/sites-available/apilb:
- access_log /var/log/nginx.access.log;
+ access_log /var/log/nginx.access.log json_combined;