Show only interval stats log during debug

[stiangrindvoll]

Displaying this log generates a lot of noise in centralized logging when
using multiple instances of statsdaemon.

Also fits better to hide this in according with the -debug flag description.

[ploxiln]

I like to see this message in the logs, to know statsdaemon is working, and receiving roughly the correct volume of stats keys from applications ... but debug level logs every single individual stat, which is way too much.

So I think what would be better is adding a log-level option, which could be [debug, info, warning, error], and you would probably prefer running at the "warning" level.

[jehiah]

@ploxiln I also use these as a sort of canary, but I think there is a need for a better way to first-class healthcheck statsdaemon to know it's sending metrics. I'm not sure the best way for that, but if such a thing exists, then moving this log to debug (or other more granular logging selections) is fine by me.