grok exporter should be able to start if file does not exist

[roidelapluie]

No description provided.

[roidelapluie]

If a file does not exist, the grok export should start. Thet it would wait with inotify for file to be present.

I propose a new metric as well: grok_pen_files_count (gauge)

[fstab]

This is definitely useful. I think it should be configurable. Maybe it is still good to keep the current behaviour as default, because if the log file is missing it is very likely a configuration error.

[fstab]

I implemented a new config option for the input section:

fail_on_missing_logfile: false

It's not released and documented yet, but it's pushed to the master branch. The grok_pending_files_count metric is still missing.