Allow sampling only for Counters

StatsD olny allows sampling on Counter metrics. To avoid creating invalid packages packages[0] this change-set adjusts the interfaces of the two mehtods in question.

[0] Illegal sampling factor for non-counter metric on line timer:0|ms|@0.050000
As discussed in email, we should keep the method signatures the same, and still perform the maybeSample, but adjust the `timingUpdate` and `gaugeUpdate` structs to drop the `sampling` parameter.