Do not silently disable metering if metering.reporter property is not set

[fr3dch3n]

I just ran into the problem that I could not figure out why a gauge was not working. Turned out I did not set metering.reporter config option and thereby silently turned off reporting as a whole. I think that "console" should be the default value for this setting instead of shutting down reporting altogether and an exception should be thrown if the config value given is unsupported (e. g. "ganglia").

Would that be acceptable behavior?