Potentially reduce the amount of memory required for Datadog publishing.

Previously this plugin with group and order all of the metrics before
sending them to DataDog.  I don't believe that this is necessary and
was causing an excess memory allocation to happen.  On VMs that have
a lot of metrics, this could cause out of memory errors.  This change
makes it so that instead of grouping and sorting the metrics, they
are just iterated over and sent directly to DataDog from the same
data structure that Yammer metrics maintains.

NOTE: This code is untested.