Separate the execution of the rsyslog exporter

[soso-bc]

I think it would be better if the exporter were managed by systemd rather than rsyslog.

rsyslog could send the stats to the exporter:
module(load="impstats" interval="10" format="json" resetCounters="off" push.url="http://<exporter>:<port>/imstats" )

The exporter would then retrieve them via its own endpoint (imstats).

This way:

• the exporter would be autonomous and could be managed by systemd
• the exporter could run in its own user context (not root)
• if there's a problem with rsyslog, the exporter would still run
• if maintenance (upgrade) is required on the exporter, there would be no need to restart rsyslog
• etc...