Add Prometheus monitoring

This change adds an alert that will trigger when Heka doesn't send data
anymore to its configured outputs. This should cover cases when Heka is
down or stuck.

Change-Id: I59b578d5366d0f5bd584eb4cccea7355765e9dc7
Depends-On: Ic13fd1b6fe4008771b93057bf2cbf0b78825516f
5 files changed