0.2.3 Release

Responding to a user-reported issue. Thanks for the feedback!

When a timer value wasn't sent within a collection interval (60 seconds) the counter and rate values would be zero, and the aggregates (sum/avg/min/max/percentiles) would be undefined. Those would cause a malformed message to the Stackdriver gateway, which would be rejected.

This patch checks for those undefined values. It also adds an option sendZeroTimersAndRates to the Stackdriver module configuration. Setting this option to false will cause zero valued counters and rates (in both counter and timer variables) to not be sent. By default, statsd (and this plugin) will continue to send those variables as zero to keep a continuous series.