Loosen Nginx error log regex matching

[Leo1003]

Since the fields can be empty in some circustances, it is better to use '*' instead of '+' to match the values.

Below is an example log from our Nginx container which can fail the parse_nginx_log() finction:

2023/09/08 13:50:28 [warn] 3#3: *531 an upstream response is buffered to a temporary file /var/lib/nginx/tmp/fastcgi/6/03/0000000036 while reading upstream, client: 140.113.168.124, server: , request: "GET / HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "duty.cs.nctu.edu.tw", referrer: "https://oauth.cs.nctu.edu.tw/"

[pront]

Hi @Leo1003, thanks for submitting this change! Can you please add a test and an entry in the CHANGELOG.md?

[Leo1003]

I have added a test and an entry in the CHANGELOG

[pront]

Thank you!