Make host field configurable

[tokred]

We use heartbeat inputs to monitor the availability of our Logstash instances.

Unfortunately, the host field is automatically set to the FQDN hostname via Socket.gethostname. Depending on the environment, it may be desireable to change this, e.g. to a certain IP for machines with multiple interfaces, or to only the host part of FQDN, etc. Current workaround is to tag heartbeat messages and apply a mutate filter in Logstash config.

Please expose the host field via configuration parameter in order to allow setting a custom value. Big thanks!