This patch adds support for integrating Prometheus with Fluentd.
This can be used to extract useful information about the status
of Fluentd, such as output buffer capacity and logging rate,
and also to extract metrics from logs via custom Fluentd
configuration. More information can be found here in [1].

[1] https://docs.fluentd.org/monitoring-fluentd/monitoring-prometheus

Change-Id: I233d6dd744848ef1f1589a462dbf272ed0f3aaae