Statsd: Don't append an origin tag if already present in sample tags

[lardcanoe]

Describe what happened:
Our statsd code passes in tags, e.g. statsd.increment "foo", tags: "region:us-west-2"
But in datadog UI, there are actual multiple region tags. One from the sample, and one from the server of the datadog agent. This makes it hard (impossible?) to properly filter.

Describe what you expected:
I would expect only one instance of the tag. Sample wins.

For a fix, I'd be fine if an env var passed to the agent controlled if it skips adding an origin tag if already present in the sample.