Disable the statsd counter for the combinebykey metric.

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.

Profiling of partitions can be done using profilePartitions(),
the metric as such does not give enough insight as is.

We will check with Mike to see if this resolves the issue.

@mredan @Shopify/data-engineers