Write gauges by default in statsd.

If a metric is not found to have a value in type map previously the
metric would be lost. This is a slightly unintuitive behaviour with
regard to the statsd reporter. Prefer instead to ship the untyped as
gauges, which has value in a statsd context.

Signed-off-by: Brian L. Troutwine <brian.troutwine@adroll.com>