fix metric_types for beanstalkd

When setting up beanstalkd with a Librato handler, I noticed that it
wasn't reporting the metric types correctly. This patch will whitelist
stats that are known to be counters (tested with beanstalkd 1.4.6) and
default to GAUGE for everything else.
fix whitespace
Merge pull request #337 from wadey/beanstalkd-metric-type

fix metric_types for beanstalkd