When following log files, tail them rather than truncate them.

[eklitzke]

This change makes the exporter tail log files, meaning that the files can be opened read only and don't need to be truncated. This seems like a general all around win because:

• It doesn't require any special syslog configuration to duplicate logs
• The exporter process can be run by a user without write privileges to log files
• This will likely work better out-of-the-box for most people

[jangrewe]

Thanks, that's a very sane idea.

[BartVerc]

Thanks for the pull request.
It looks like you are still using the duplicated log file since the default path to the log is the duplicated one.
Can you fix it so that the configured Postfix log file is used by default?

[BartVerc]

Thanks for the work, it should have been merged earlier.