Add configuration for log level and format

[mcinquin]

Hello,

Can you add an option to configure the log level and format like in other Prometheus exporters ?

eg.

      --log.level="info"  Only log messages with the given severity or above. Valid levels: [debug, info, warn, error, fatal]
      --log.format="logger:stderr"
                          Set the log target and format. Example: "logger:syslog?appname=bob&local=7" or "logger:stdout?json=true"

Regards.

Aha! Link: https://nginx.aha.io/features/IC-305