StatsD: support negative counter values or allow suppressing of error logging

Hello,
I'm using telegraf as a StatsD backend and my application frequently logs negative counter values, eg. `foo:-1|c`.
I do not particularly depend on these values being supported, but my log files grow rapidly because telegraf keeps logging `Error: +- values are only supported for gauges: foo:-1|c`.

Can this log message be suppressed, or alternatively, can negative counters be supported?
