Set CombineByKey statsd metric at sample rate of 0.001

Mike Redan notified us that one of our statsd metrics was
emitting 100's of values and it seems most like it was caused
by the statsd counter in the combineByKey function.
forgot this line
Merge pull request #4 from Shopify/disable_combinebykey_metric

Introduce sample rate for the statsd counter for the combinebykey metric