Add support for disabling DogStatsD payloads from being forwarded.

[tobz]

Context

In the Datadog Agent, the forwarding of specific types of payloads -- events, service checks, series, sketches, and more -- can be disabled through the configuration, as seen here.

This is, in generally, extremely niche, but we'll want to make sure we support it.

Notes

The Datadog Agent handles this in the flush/serialization phase, but we should figure out if there's any good reason to even bother creating the events and shuffling them through the topology, since it would be vastly more efficient to just drop them before sending them out of the DogStatsD source.