Add custom field for time

[dolgovas]

Right now in log we can use these variables

• time-iso8601
• time-local
• msec

However, none of them contain a useful time format with milliseconds. When users want to add milliseconds in their logs (for example, in ELK), they need to make customizations for time, like in this example:

map "$time_local:$msec" $time_local_ms { ~(^\S+)(\s+\S+):\d+\.(\d+)$
    $1.$3$2; }

I think it's a good idea to add at least milliseconds in time-iso8601 or at maximum provide a fully custom time variable like in GNU date.