Include additional performance data

[shakalandy]

It would be great to have the following counters included in your plugin as performance data - with different Warning/Critical barriers as Index errors:

• unprocessed messages (in Case Graylog cannot write to ES, or has performance problems due to bad pipelines, grok patterns, etc...
• journal size in percent (same reason as above)
• Input Buffer, Process Buffer, Output Buffer

Of course, everything optional - but for larger installations a tracking of each metric would be very useful.

[catinello]

PR's are welcome! :-)